[PATCH 0/7] mm: Page fault enhancements

[PATCH 0/7] mm: Page fault enhancements

[Peter Xu]

This series is split out of userfaultfd-wp series to only cover the
general page fault changes, since it seems to make sense itself.

Basically it does two things:

  (a) Allows the page fault handlers to be more interactive on not
      only SIGKILL, but also the rest of userspace signals (especially
      for user-mode faults), and,

  (b) Allows the page fault retry (VM_FAULT_RETRY) to happen for more
      than once.

I'm keeping the CC list as in uffd-wp v5, hopefully I'm not sending
too much spams...

And, instead of writting again the cover letter, I'm just copy-pasting
my previous link here which has more details on why we do this:

  https://patchwork.kernel.org/cover/10691991/

The major change from that latest version should be that we introduced
a new page fault flag FAULT_FLAG_INTERRUPTIBLE as suggested by Linus
[1] to represents that we would like the fault handler to respond to
non-fatal signals.  Also, we're more careful now on when to do the
immediate return of the page fault for such signals.  For example, now
we'll only check against signal_pending() for user-mode page faults
and we keep the kernel-mode page fault patch untouched for it.  More
information can be found in separate patches.

The patchset is only lightly tested on x86.

All comments are greatly welcomed.  Thanks,

[1] https://lkml.org/lkml/2019/6/25/1382

Peter Xu (7):
  mm/gup: Rename "nonblocking" to "locked" where proper
  mm: Introduce FAULT_FLAG_DEFAULT
  mm: Introduce FAULT_FLAG_INTERRUPTIBLE
  mm: Return faster for non-fatal signals in user mode faults
  userfaultfd: Don't retake mmap_sem to emulate NOPAGE
  mm: Allow VM_FAULT_RETRY for multiple times
  mm/gup: Allow VM_FAULT_RETRY for multiple times

 arch/alpha/mm/fault.c           |  7 +--
 arch/arc/mm/fault.c             | 17 ++-----
 arch/arm/mm/fault.c             | 14 +++---
 arch/arm64/mm/fault.c           | 16 +++----
 arch/hexagon/mm/vm_fault.c      |  6 +--
 arch/ia64/mm/fault.c            |  6 +--
 arch/m68k/mm/fault.c            | 10 ++--
 arch/microblaze/mm/fault.c      |  6 +--
 arch/mips/mm/fault.c            |  6 +--
 arch/nds32/mm/fault.c           | 12 ++---
 arch/nios2/mm/fault.c           |  8 ++--
 arch/openrisc/mm/fault.c        |  6 +--
 arch/parisc/mm/fault.c          |  9 ++--
 arch/powerpc/mm/fault.c         | 10 ++--
 arch/riscv/mm/fault.c           | 12 ++---
 arch/s390/mm/fault.c            | 11 ++---
 arch/sh/mm/fault.c              |  7 ++-
 arch/sparc/mm/fault_32.c        |  5 +-
 arch/sparc/mm/fault_64.c        |  6 +--
 arch/um/kernel/trap.c           |  7 +--
 arch/unicore32/mm/fault.c       | 11 ++---
 arch/x86/mm/fault.c             |  6 +--
 arch/xtensa/mm/fault.c          |  6 +--
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 +++--
 fs/userfaultfd.c                | 28 +-----------
 include/linux/mm.h              | 81 +++++++++++++++++++++++++++++----
 include/linux/sched/signal.h    | 12 +++++
 mm/filemap.c                    |  2 +-
 mm/gup.c                        | 61 ++++++++++++++-----------
 mm/hugetlb.c                    | 14 +++---
 mm/shmem.c                      |  2 +-
 31 files changed, 226 insertions(+), 190 deletions(-)

-- 
2.21.0

[PATCH 1/7] mm/gup: Rename "nonblocking" to "locked" where proper

[Peter Xu]

There's plenty of places around __get_user_pages() that has a parameter
"nonblocking" which does not really mean that "it won't block" (because
it can really block) but instead it shows whether the mmap_sem is
released by up_read() during the page fault handling mostly when
VM_FAULT_RETRY is returned.

We have the correct naming in e.g. get_user_pages_locked() or
get_user_pages_remote() as "locked", however there're still many places
that are using the "nonblocking" as name.

Renaming the places to "locked" where proper to better suite the
functionality of the variable.  While at it, fixing up some of the
comments accordingly.

Reviewed-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c     | 44 +++++++++++++++++++++-----------------------
 mm/hugetlb.c |  8 ++++----
 2 files changed, 25 insertions(+), 27 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ddde097cf9e4..58d282115d9b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -625,12 +625,12 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 }
 
 /*
- * mmap_sem must be held on entry.  If @nonblocking != NULL and
- * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.
- * If it is, *@nonblocking will be set to 0 and -EBUSY returned.
+ * mmap_sem must be held on entry.  If @locked != NULL and *@flags
+ * does not include FOLL_NOWAIT, the mmap_sem may be released.  If it
+ * is, *@locked will be set to 0 and -EBUSY returned.
  */
 static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
-		unsigned long address, unsigned int *flags, int *nonblocking)
+		unsigned long address, unsigned int *flags, int *locked)
 {
 	unsigned int fault_flags = 0;
 	vm_fault_t ret;
@@ -642,7 +642,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
-	if (nonblocking)
+	if (locked)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
@@ -668,8 +668,8 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	}
 
 	if (ret & VM_FAULT_RETRY) {
-		if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
-			*nonblocking = 0;
+		if (locked && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
+			*locked = 0;
 		return -EBUSY;
 	}
 
@@ -746,7 +746,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  *		only intends to ensure the pages are faulted in.
  * @vmas:	array of pointers to vmas corresponding to each page.
  *		Or NULL if the caller does not require them.
- * @nonblocking: whether waiting for disk IO or mmap_sem contention
+ * @locked:     whether we're still with the mmap_sem held
  *
  * Returns number of pages pinned. This may be fewer than the number
  * requested. If nr_pages is 0 or negative, returns 0. If no pages
@@ -775,13 +775,11 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  * appropriate) must be called after the page is finished with, and
  * before put_page is called.
  *
- * If @nonblocking != NULL, __get_user_pages will not wait for disk IO
- * or mmap_sem contention, and if waiting is needed to pin all pages,
- * *@nonblocking will be set to 0.  Further, if @gup_flags does not
- * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in
- * this case.
+ * If @locked != NULL, *@locked will be set to 0 when mmap_sem is
+ * released by an up_read().  That can happen if @gup_flags does not
+ * have FOLL_NOWAIT.
  *
- * A caller using such a combination of @nonblocking and @gup_flags
+ * A caller using such a combination of @locked and @gup_flags
  * must therefore hold the mmap_sem for reading only, and recognize
  * when it's been released.  Otherwise, it must be held for either
  * reading or writing and will not be released.
@@ -793,7 +791,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
-		struct vm_area_struct **vmas, int *nonblocking)
+		struct vm_area_struct **vmas, int *locked)
 {
 	long ret = 0, i = 0;
 	struct vm_area_struct *vma = NULL;
@@ -837,7 +835,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			if (is_vm_hugetlb_page(vma)) {
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
-						gup_flags, nonblocking);
+						gup_flags, locked);
 				continue;
 			}
 		}
@@ -855,7 +853,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		page = follow_page_mask(vma, start, foll_flags, &ctx);
 		if (!page) {
 			ret = faultin_page(tsk, vma, start, &foll_flags,
-					nonblocking);
+					   locked);
 			switch (ret) {
 			case 0:
 				goto retry;
@@ -1508,7 +1506,7 @@ EXPORT_SYMBOL(get_user_pages);
  * @vma:   target vma
  * @start: start address
  * @end:   end address
- * @nonblocking:
+ * @locked: whether the mmap_sem is still held
  *
  * This takes care of mlocking the pages too if VM_LOCKED is set.
  *
@@ -1516,14 +1514,14 @@ EXPORT_SYMBOL(get_user_pages);
  *
  * vma->vm_mm->mmap_sem must be held.
  *
- * If @nonblocking is NULL, it may be held for read or write and will
+ * If @locked is NULL, it may be held for read or write and will
  * be unperturbed.
  *
- * If @nonblocking is non-NULL, it must held for read only and may be
- * released.  If it's released, *@nonblocking will be set to 0.
+ * If @locked is non-NULL, it must held for read only and may be
+ * released.  If it's released, *@locked will be set to 0.
  */
 long populate_vma_page_range(struct vm_area_struct *vma,
-		unsigned long start, unsigned long end, int *nonblocking)
+		unsigned long start, unsigned long end, int *locked)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long nr_pages = (end - start) / PAGE_SIZE;
@@ -1558,7 +1556,7 @@ long populate_vma_page_range(struct vm_area_struct *vma,
 	 * not result in a stack expansion that recurses back here.
 	 */
 	return __get_user_pages(current, mm, start, nr_pages, gup_flags,
-				NULL, NULL, nonblocking);
+				NULL, NULL, locked);
 }
 
 /*
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac843d32b019..ba179c2fa8fb 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4240,7 +4240,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			 struct page **pages, struct vm_area_struct **vmas,
 			 unsigned long *position, unsigned long *nr_pages,
-			 long i, unsigned int flags, int *nonblocking)
+			 long i, unsigned int flags, int *locked)
 {
 	unsigned long pfn_offset;
 	unsigned long vaddr = *position;
@@ -4311,7 +4311,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				spin_unlock(ptl);
 			if (flags & FOLL_WRITE)
 				fault_flags |= FAULT_FLAG_WRITE;
-			if (nonblocking)
+			if (locked)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY;
 			if (flags & FOLL_NOWAIT)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
@@ -4328,9 +4328,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				break;
 			}
 			if (ret & VM_FAULT_RETRY) {
-				if (nonblocking &&
+				if (locked &&
 				    !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
-					*nonblocking = 0;
+					*locked = 0;
 				*nr_pages = 0;
 				/*
 				 * VM_FAULT_RETRY must not return an
-- 
2.21.0

[PATCH 2/7] mm: Introduce FAULT_FLAG_DEFAULT

[Peter Xu]

Although there're tons of arch-specific page fault handlers, most of
them are still sharing the same initial value of the page fault flags.
Say, merely all of the page fault handlers would allow the fault to be
retried, and they also allow the fault to respond to SIGKILL.

Let's define a default value for the fault flags to replace those
initial page fault flags that were copied over.  With this, it'll be
far easier to introduce new fault flag that can be used by all the
architectures instead of touching all the archs.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/alpha/mm/fault.c      | 2 +-
 arch/arc/mm/fault.c        | 2 +-
 arch/arm/mm/fault.c        | 2 +-
 arch/arm64/mm/fault.c      | 2 +-
 arch/hexagon/mm/vm_fault.c | 2 +-
 arch/ia64/mm/fault.c       | 2 +-
 arch/m68k/mm/fault.c       | 2 +-
 arch/microblaze/mm/fault.c | 2 +-
 arch/mips/mm/fault.c       | 2 +-
 arch/nds32/mm/fault.c      | 2 +-
 arch/nios2/mm/fault.c      | 2 +-
 arch/openrisc/mm/fault.c   | 2 +-
 arch/parisc/mm/fault.c     | 2 +-
 arch/powerpc/mm/fault.c    | 2 +-
 arch/riscv/mm/fault.c      | 2 +-
 arch/s390/mm/fault.c       | 2 +-
 arch/sh/mm/fault.c         | 2 +-
 arch/sparc/mm/fault_32.c   | 2 +-
 arch/sparc/mm/fault_64.c   | 2 +-
 arch/um/kernel/trap.c      | 2 +-
 arch/unicore32/mm/fault.c  | 2 +-
 arch/x86/mm/fault.c        | 2 +-
 arch/xtensa/mm/fault.c     | 2 +-
 include/linux/mm.h         | 7 +++++++
 24 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c
index 188fc9256baf..fceb684d3090 100644
--- a/arch/alpha/mm/fault.c
+++ b/arch/alpha/mm/fault.c
@@ -89,7 +89,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	const struct exception_table_entry *fixup;
 	int si_code = SEGV_MAPERR;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	/* As of EV6, a load into $31/$f31 is a prefetch, and never faults
 	   (or is suppressed by the PALcode).  Support that for older CPUs
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 8cca03480bb2..9f9b55d431ac 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -67,7 +67,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 	int ret;
 	vm_fault_t fault;
 	int write = regs->ecr_cause & ECR_C_PROTV_STORE;  /* ST/EX */
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	/*
 	 * We fault-in kernel-space virtual memory on-demand. The
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 0048eadd0681..d98af3169d69 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -261,7 +261,7 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	struct mm_struct *mm;
 	int sig, code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	if (notify_page_fault(regs, fsr))
 		return 0;
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 2d115016feb4..1cdd68e1c63c 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -433,7 +433,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	struct mm_struct *mm;
 	vm_fault_t fault, major = 0;
 	unsigned long vm_flags = VM_READ | VM_WRITE;
-	unsigned int mm_flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int mm_flags = FAULT_FLAG_DEFAULT;
 
 	if (notify_page_fault(regs, esr))
 		return 0;
diff --git a/arch/hexagon/mm/vm_fault.c b/arch/hexagon/mm/vm_fault.c
index b7a99aa5b0ba..c0411412ce3c 100644
--- a/arch/hexagon/mm/vm_fault.c
+++ b/arch/hexagon/mm/vm_fault.c
@@ -41,7 +41,7 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 	int si_code = SEGV_MAPERR;
 	vm_fault_t fault;
 	const struct exception_table_entry *fixup;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	/*
 	 * If we're in an interrupt or have no user context,
diff --git a/arch/ia64/mm/fault.c b/arch/ia64/mm/fault.c
index 5baeb022f474..b01588e18079 100644
--- a/arch/ia64/mm/fault.c
+++ b/arch/ia64/mm/fault.c
@@ -87,7 +87,7 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	struct mm_struct *mm = current->mm;
 	unsigned long mask;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	mask = ((((isr >> IA64_ISR_X_BIT) & 1UL) << VM_EXEC_BIT)
 		| (((isr >> IA64_ISR_W_BIT) & 1UL) << VM_WRITE_BIT));
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 9b6163c05a75..22e4e6c68bfe 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -71,7 +71,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct * vma;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	pr_debug("do page fault:\nregs->sr=%#x, regs->pc=%#lx, address=%#lx, %ld, %p\n",
 		regs->sr, regs->pc, address, error_code, mm ? mm->pgd : NULL);
diff --git a/arch/microblaze/mm/fault.c b/arch/microblaze/mm/fault.c
index 202ad6a494f5..fff0e08ec7c6 100644
--- a/arch/microblaze/mm/fault.c
+++ b/arch/microblaze/mm/fault.c
@@ -91,7 +91,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	int code = SEGV_MAPERR;
 	int is_write = error_code & ESR_S;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	regs->ear = address;
 	regs->esr = error_code;
diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
index 73d8a0f0b810..5879dd93bf34 100644
--- a/arch/mips/mm/fault.c
+++ b/arch/mips/mm/fault.c
@@ -44,7 +44,7 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	const int field = sizeof(unsigned long) * 2;
 	int si_code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	static DEFINE_RATELIMIT_STATE(ratelimit_state, 5 * HZ, 10);
 
diff --git a/arch/nds32/mm/fault.c b/arch/nds32/mm/fault.c
index 68d5f2a27f38..875e254aaa7c 100644
--- a/arch/nds32/mm/fault.c
+++ b/arch/nds32/mm/fault.c
@@ -76,7 +76,7 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 	int si_code;
 	vm_fault_t fault;
 	unsigned int mask = VM_READ | VM_WRITE | VM_EXEC;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	error_code = error_code & (ITYPE_mskINST | ITYPE_mskETYPE);
 	tsk = current;
diff --git a/arch/nios2/mm/fault.c b/arch/nios2/mm/fault.c
index 6a2e716b959f..a401b45cae47 100644
--- a/arch/nios2/mm/fault.c
+++ b/arch/nios2/mm/fault.c
@@ -47,7 +47,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	struct mm_struct *mm = tsk->mm;
 	int code = SEGV_MAPERR;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	cause >>= 2;
 
diff --git a/arch/openrisc/mm/fault.c b/arch/openrisc/mm/fault.c
index 9eee5bf3db27..ebdddcc1de60 100644
--- a/arch/openrisc/mm/fault.c
+++ b/arch/openrisc/mm/fault.c
@@ -50,7 +50,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 	struct vm_area_struct *vma;
 	int si_code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 
diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
index c8e8b7c05558..48cc1305850a 100644
--- a/arch/parisc/mm/fault.c
+++ b/arch/parisc/mm/fault.c
@@ -273,7 +273,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 	if (!mm)
 		goto no_context;
 
-	flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	flags = FAULT_FLAG_DEFAULT;
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index ec6b7ad70659..6d86d6b52b73 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -456,7 +456,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 {
 	struct vm_area_struct * vma;
 	struct mm_struct *mm = current->mm;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
  	int is_exec = TRAP(regs) == 0x400;
 	int is_user = user_mode(regs);
 	int is_write = page_fault_is_write(error_code);
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 3e2708c626a8..a6b26c588ae6 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -28,7 +28,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	struct vm_area_struct *vma;
 	struct mm_struct *mm;
 	unsigned long addr, cause;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 	int code = SEGV_MAPERR;
 	vm_fault_t fault;
 
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index df75d574246d..1717163e850a 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -442,7 +442,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 
 	address = trans_exc_code & __FAIL_ADDR_MASK;
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
-	flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	flags = FAULT_FLAG_DEFAULT;
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 	if (access == VM_WRITE || (trans_exc_code & store_indication) == 0x400)
diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c
index 6defd2c6d9b1..5ca9f80f18d3 100644
--- a/arch/sh/mm/fault.c
+++ b/arch/sh/mm/fault.c
@@ -397,7 +397,7 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 	struct mm_struct *mm;
 	struct vm_area_struct * vma;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;
diff --git a/arch/sparc/mm/fault_32.c b/arch/sparc/mm/fault_32.c
index b0440b0edd97..a2c9610e1666 100644
--- a/arch/sparc/mm/fault_32.c
+++ b/arch/sparc/mm/fault_32.c
@@ -168,7 +168,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	int from_user = !(regs->psr & PSR_PS);
 	int code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	if (text_fault)
 		address = regs->pc;
diff --git a/arch/sparc/mm/fault_64.c b/arch/sparc/mm/fault_64.c
index 8f8a604c1300..89a3a7913854 100644
--- a/arch/sparc/mm/fault_64.c
+++ b/arch/sparc/mm/fault_64.c
@@ -281,7 +281,7 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 	int si_code, fault_code;
 	vm_fault_t fault;
 	unsigned long address, mm_rss;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	fault_code = get_thread_fault_code();
 
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 0e8b6158f224..9342ff15ac0d 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -32,7 +32,7 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 	pmd_t *pmd;
 	pte_t *pte;
 	int err = -EFAULT;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	*code_out = SEGV_MAPERR;
 
diff --git a/arch/unicore32/mm/fault.c b/arch/unicore32/mm/fault.c
index 33e0d8a267e8..0516c72ae702 100644
--- a/arch/unicore32/mm/fault.c
+++ b/arch/unicore32/mm/fault.c
@@ -201,7 +201,7 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	struct mm_struct *mm;
 	int sig, code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 46df4c6aae46..0b0d7737972a 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1305,7 +1305,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 	struct task_struct *tsk;
 	struct mm_struct *mm;
 	vm_fault_t fault, major = 0;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;
diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index 2ab0e0dcd166..d16b0607f147 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -43,7 +43,7 @@ void do_page_fault(struct pt_regs *regs)
 
 	int is_write, is_exec;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	code = SEGV_MAPERR;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd0b5f4e1e45..8eb796f2fa19 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -393,6 +393,13 @@ extern pgprot_t protection_map[16];
 #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
 #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
 
+/*
+ * The default fault flags that should be used by most of the
+ * arch-specific page fault handlers.
+ */
+#define FAULT_FLAG_DEFAULT  (FAULT_FLAG_ALLOW_RETRY | \
+			     FAULT_FLAG_KILLABLE)
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \
-- 
2.21.0

[PATCH 3/7] mm: Introduce FAULT_FLAG_INTERRUPTIBLE

[Peter Xu]

handle_userfaultfd() is currently the only one place in the kernel
page fault procedures that can respond to non-fatal userspace signals.
It was trying to detect such an allowance by checking against USER &
KILLABLE flags, which was "un-official".

In this patch, we introduced a new flag (FAULT_FLAG_INTERRUPTIBLE) to
show that the fault handler allows the fault procedure to respond even
to non-fatal signals.  Meanwhile, add this new flag to the default
fault flags so that all the page fault handlers can benefit from the
new flag.  With that, replacing the userfault check to this one.

Since the line is getting even longer, clean up the fault flags a bit
too to ease TTY users.

Although we've got a new flag and applied it, we shouldn't have any
functional change with this patch so far.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 fs/userfaultfd.c   |  4 +---
 include/linux/mm.h | 39 ++++++++++++++++++++++++++++-----------
 2 files changed, 29 insertions(+), 14 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ae0b8b5f69e6..50a21fba25d2 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -452,9 +452,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	uwq.ctx = ctx;
 	uwq.waken = false;
 
-	return_to_userland =
-		(vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
-		(FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
+	return_to_userland = vmf->flags & FAULT_FLAG_INTERRUPTIBLE;
 	blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
 			 TASK_KILLABLE;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8eb796f2fa19..845a4f5153dd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -383,22 +383,38 @@ extern unsigned int kobjsize(const void *objp);
  */
 extern pgprot_t protection_map[16];
 
-#define FAULT_FLAG_WRITE	0x01	/* Fault was a write access */
-#define FAULT_FLAG_MKWRITE	0x02	/* Fault was mkwrite of existing pte */
-#define FAULT_FLAG_ALLOW_RETRY	0x04	/* Retry fault if blocking */
-#define FAULT_FLAG_RETRY_NOWAIT	0x08	/* Don't drop mmap_sem and wait when retrying */
-#define FAULT_FLAG_KILLABLE	0x10	/* The fault task is in SIGKILL killable region */
-#define FAULT_FLAG_TRIED	0x20	/* Second try */
-#define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
-#define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
-#define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
+/**
+ * Fault flag definitions.
+ *
+ * @FAULT_FLAG_WRITE: Fault was a write fault.
+ * @FAULT_FLAG_MKWRITE: Fault was mkwrite of existing PTE.
+ * @FAULT_FLAG_ALLOW_RETRY: Allow to retry the fault if blocked.
+ * @FAULT_FLAG_RETRY_NOWAIT: Don't drop mmap_sem and wait when retrying.
+ * @FAULT_FLAG_KILLABLE: The fault task is in SIGKILL killable region.
+ * @FAULT_FLAG_TRIED: The fault has been tried once.
+ * @FAULT_FLAG_USER: The fault originated in userspace.
+ * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
+ * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
+ * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ */
+#define FAULT_FLAG_WRITE			0x01
+#define FAULT_FLAG_MKWRITE			0x02
+#define FAULT_FLAG_ALLOW_RETRY			0x04
+#define FAULT_FLAG_RETRY_NOWAIT			0x08
+#define FAULT_FLAG_KILLABLE			0x10
+#define FAULT_FLAG_TRIED			0x20
+#define FAULT_FLAG_USER				0x40
+#define FAULT_FLAG_REMOTE			0x80
+#define FAULT_FLAG_INSTRUCTION  		0x100
+#define FAULT_FLAG_INTERRUPTIBLE		0x200
 
 /*
  * The default fault flags that should be used by most of the
  * arch-specific page fault handlers.
  */
 #define FAULT_FLAG_DEFAULT  (FAULT_FLAG_ALLOW_RETRY | \
-			     FAULT_FLAG_KILLABLE)
+			     FAULT_FLAG_KILLABLE | \
+			     FAULT_FLAG_INTERRUPTIBLE)
 
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
@@ -409,7 +425,8 @@ extern pgprot_t protection_map[16];
 	{ FAULT_FLAG_TRIED,		"TRIED" }, \
 	{ FAULT_FLAG_USER,		"USER" }, \
 	{ FAULT_FLAG_REMOTE,		"REMOTE" }, \
-	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }
+	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }, \
+	{ FAULT_FLAG_INTERRUPTIBLE,	"INTERRUPTIBLE" }
 
 /*
  * vm_fault is filled by the the pagefault handler and passed to the vma's
-- 
2.21.0

[PATCH 4/7] mm: Return faster for non-fatal signals in user mode faults

[Peter Xu]

The idea comes from the upstream discussion between Linus and Andrea:

  https://lkml.org/lkml/2017/10/30/560

A summary to the issue: there was a special path in handle_userfault()
in the past that we'll return a VM_FAULT_NOPAGE when we detected
non-fatal signals when waiting for userfault handling.  We did that by
reacquiring the mmap_sem before returning.  However that brings a risk
in that the vmas might have changed when we retake the mmap_sem and
even we could be holding an invalid vma structure.

This patch is a preparation of removing that special path by allowing
the page fault to return even faster if we were interrupted by a
non-fatal signal during a user-mode page fault handling routine.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/alpha/mm/fault.c        |  3 ++-
 arch/arc/mm/fault.c          | 14 ++++----------
 arch/arm/mm/fault.c          |  9 +++++----
 arch/arm64/mm/fault.c        |  9 +++++----
 arch/hexagon/mm/vm_fault.c   |  3 ++-
 arch/ia64/mm/fault.c         |  3 ++-
 arch/m68k/mm/fault.c         |  5 +++--
 arch/microblaze/mm/fault.c   |  3 ++-
 arch/mips/mm/fault.c         |  3 ++-
 arch/nds32/mm/fault.c        |  9 +++++----
 arch/nios2/mm/fault.c        |  3 ++-
 arch/openrisc/mm/fault.c     |  3 ++-
 arch/parisc/mm/fault.c       |  3 ++-
 arch/powerpc/mm/fault.c      |  2 ++
 arch/riscv/mm/fault.c        |  5 +++--
 arch/s390/mm/fault.c         |  4 ++--
 arch/sh/mm/fault.c           |  4 ++++
 arch/sparc/mm/fault_32.c     |  2 +-
 arch/sparc/mm/fault_64.c     |  3 ++-
 arch/um/kernel/trap.c        |  4 +++-
 arch/unicore32/mm/fault.c    |  5 +++--
 arch/x86/mm/fault.c          |  2 ++
 arch/xtensa/mm/fault.c       |  3 ++-
 include/linux/sched/signal.h | 12 ++++++++++++
 24 files changed, 74 insertions(+), 42 deletions(-)

diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c
index fceb684d3090..adf5c46ddb68 100644
--- a/arch/alpha/mm/fault.c
+++ b/arch/alpha/mm/fault.c
@@ -150,7 +150,8 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 9f9b55d431ac..49b6a6802862 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -136,17 +136,11 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if (fatal_signal_pending(current)) {
-
-		/*
-		 * if fault retry, mmap_sem already relinquished by core mm
-		 * so OK to return to user mode (with signal handled first)
-		 */
-		if (fault & VM_FAULT_RETRY) {
-			if (!user_mode(regs))
-				goto no_context;
+	if (unlikely((fault & VM_FAULT_RETRY) && signal_pending(current))) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
+			goto no_context;
+		if (user_mode(regs))
 			return;
-		}
 	}
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index d98af3169d69..9af92c1c5510 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -311,14 +311,15 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_page_fault(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if (unlikely(fault & VM_FAULT_RETRY && signal_pending(current))) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return 0;
+		if (user_mode(regs))
+			return 0;
 	}
 
 	/*
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 1cdd68e1c63c..40cb184068f2 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -502,15 +502,16 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 
 	if (fault & VM_FAULT_RETRY) {
 		/*
-		 * If we need to retry but a fatal signal is pending,
+		 * If we need to retry but a signal is pending, try to
 		 * handle the signal first. We do not need to release
 		 * the mmap_sem because it would already be released
 		 * in __lock_page_or_retry in mm/filemap.c.
 		 */
-		if (fatal_signal_pending(current)) {
-			if (!user_mode(regs))
+		if (signal_pending(current)) {
+			if (fatal_signal_pending(current) && !user_mode(regs))
 				goto no_context;
-			return 0;
+			if (user_mode(regs))
+				return 0;
 		}
 
 		/*
diff --git a/arch/hexagon/mm/vm_fault.c b/arch/hexagon/mm/vm_fault.c
index c0411412ce3c..e051564b5e33 100644
--- a/arch/hexagon/mm/vm_fault.c
+++ b/arch/hexagon/mm/vm_fault.c
@@ -91,7 +91,8 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	/* The most common case -- we are done. */
diff --git a/arch/ia64/mm/fault.c b/arch/ia64/mm/fault.c
index b01588e18079..54fc86b4f418 100644
--- a/arch/ia64/mm/fault.c
+++ b/arch/ia64/mm/fault.c
@@ -163,7 +163,8 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 22e4e6c68bfe..6376c079ea54 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -138,8 +138,9 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	fault = handle_mm_fault(vma, address, flags);
 	pr_debug("handle_mm_fault returns %x\n", fault);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
-		return 0;
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
+		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
diff --git a/arch/microblaze/mm/fault.c b/arch/microblaze/mm/fault.c
index fff0e08ec7c6..03c6a4cdf66c 100644
--- a/arch/microblaze/mm/fault.c
+++ b/arch/microblaze/mm/fault.c
@@ -217,7 +217,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
index 5879dd93bf34..f0b34674a01f 100644
--- a/arch/mips/mm/fault.c
+++ b/arch/mips/mm/fault.c
@@ -154,7 +154,8 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
diff --git a/arch/nds32/mm/fault.c b/arch/nds32/mm/fault.c
index 875e254aaa7c..d203d9b0f7a0 100644
--- a/arch/nds32/mm/fault.c
+++ b/arch/nds32/mm/fault.c
@@ -206,14 +206,15 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if ((fault & VM_FAULT_RETRY) && signal_pending(current)) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return;
+		if (user_mode(regs))
+			return;
 	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/nios2/mm/fault.c b/arch/nios2/mm/fault.c
index a401b45cae47..f9f178484184 100644
--- a/arch/nios2/mm/fault.c
+++ b/arch/nios2/mm/fault.c
@@ -133,7 +133,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/openrisc/mm/fault.c b/arch/openrisc/mm/fault.c
index ebdddcc1de60..26150520bee4 100644
--- a/arch/openrisc/mm/fault.c
+++ b/arch/openrisc/mm/fault.c
@@ -161,7 +161,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
index 48cc1305850a..46a0fcecabe1 100644
--- a/arch/parisc/mm/fault.c
+++ b/arch/parisc/mm/fault.c
@@ -303,7 +303,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 6d86d6b52b73..bf06a407d1a7 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -616,6 +616,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 			 */
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if (is_user && signal_pending(current))
+				return 0;
 			if (!fatal_signal_pending(current))
 				goto retry;
 		}
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index a6b26c588ae6..ddd43d9ae45e 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -111,11 +111,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(tsk))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 1717163e850a..62235ec77b03 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -493,8 +493,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	 * the fault.
 	 */
 	fault = handle_mm_fault(vma, address, flags);
-	/* No reason to continue if interrupted by SIGKILL. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs))) {
 		fault = VM_FAULT_SIGNAL;
 		if (flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_up;
diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c
index 5ca9f80f18d3..be559fcaa3ca 100644
--- a/arch/sh/mm/fault.c
+++ b/arch/sh/mm/fault.c
@@ -506,6 +506,10 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
+
+			if (user_mode(regs) && signal_pending(tsk))
+				return;
+
 			goto retry;
 		}
 	}
diff --git a/arch/sparc/mm/fault_32.c b/arch/sparc/mm/fault_32.c
index a2c9610e1666..9bcbfcb1b8c3 100644
--- a/arch/sparc/mm/fault_32.c
+++ b/arch/sparc/mm/fault_32.c
@@ -237,7 +237,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fault_should_check_signal(from_user))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/sparc/mm/fault_64.c b/arch/sparc/mm/fault_64.c
index 89a3a7913854..1c9d51b62222 100644
--- a/arch/sparc/mm/fault_64.c
+++ b/arch/sparc/mm/fault_64.c
@@ -435,7 +435,8 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(flags & FAULT_FLAG_USER))
 		goto exit_exception;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 9342ff15ac0d..3eaf37a5539f 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -76,7 +76,9 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 
 		fault = handle_mm_fault(vma, address, flags);
 
-		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+
+		if ((fault & VM_FAULT_RETRY) &&
+		    fault_should_check_signal(is_user))
 			goto out_nosemaphore;
 
 		if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/arch/unicore32/mm/fault.c b/arch/unicore32/mm/fault.c
index 0516c72ae702..185c9b1a7a55 100644
--- a/arch/unicore32/mm/fault.c
+++ b/arch/unicore32/mm/fault.c
@@ -245,11 +245,12 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_pf(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return 0;
 
 	if (!(fault & VM_FAULT_ERROR) && (flags & FAULT_FLAG_ALLOW_RETRY)) {
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 0b0d7737972a..f1b5df6758e5 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1467,6 +1467,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
+				return;
 			if (!fatal_signal_pending(tsk))
 				goto retry;
 		}
diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index d16b0607f147..b9c2ea0de49b 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -110,7 +110,8 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 38a0f0785323..a22ad4f25072 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -374,6 +374,18 @@ static inline int signal_pending_state(long state, struct task_struct *p)
 	return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
 }
 
+/*
+ * This should only be used in fault handlers to decide whether we
+ * should stop the current fault routine to handle the signals
+ * instead.  It should normally be used when a signal interrupted a
+ * page fault which can lead to a VM_FAULT_RETRY.
+ */
+static inline bool fault_should_check_signal(bool is_user)
+{
+	return (fatal_signal_pending(current) ||
+		(is_user && signal_pending(current)));
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.
-- 
2.21.0

[PATCH 5/7] userfaultfd: Don't retake mmap_sem to emulate NOPAGE

[Peter Xu]

The idea comes from the upstream discussion between Linus and Andrea:

https://lkml.org/lkml/2017/10/30/560

A summary to the issue: there was a special path in handle_userfault()
in the past that we'll return a VM_FAULT_NOPAGE when we detected
non-fatal signals when waiting for userfault handling.  We did that by
reacquiring the mmap_sem before returning.  However that brings a risk
in that the vmas might have changed when we retake the mmap_sem and
even we could be holding an invalid vma structure.

This patch removes the risk path in handle_userfault() then we will be
sure that the callers of handle_mm_fault() will know that the VMAs
might have changed.  Meanwhile with previous patch we don't lose
responsiveness as well since the core mm code now can handle the
nonfatal userspace signals quickly even if we return VM_FAULT_RETRY.

Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 fs/userfaultfd.c | 24 ------------------------
 1 file changed, 24 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 50a21fba25d2..8b051bae97ef 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -512,30 +512,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	__set_current_state(TASK_RUNNING);
 
-	if (return_to_userland) {
-		if (signal_pending(current) &&
-		    !fatal_signal_pending(current)) {
-			/*
-			 * If we got a SIGSTOP or SIGCONT and this is
-			 * a normal userland page fault, just let
-			 * userland return so the signal will be
-			 * handled and gdb debugging works.  The page
-			 * fault code immediately after we return from
-			 * this function is going to release the
-			 * mmap_sem and it's not depending on it
-			 * (unlike gup would if we were not to return
-			 * VM_FAULT_RETRY).
-			 *
-			 * If a fatal signal is pending we still take
-			 * the streamlined VM_FAULT_RETRY failure path
-			 * and there's no need to retake the mmap_sem
-			 * in such case.
-			 */
-			down_read(&mm->mmap_sem);
-			ret = VM_FAULT_NOPAGE;
-		}
-	}
-
 	/*
 	 * Here we race with the list_del; list_add in
 	 * userfaultfd_ctx_read(), however because we don't ever run
-- 
2.21.0

[PATCH 6/7] mm: Allow VM_FAULT_RETRY for multiple times

[Peter Xu]

The idea comes from a discussion between Linus and Andrea [1].

Before this patch we only allow a page fault to retry once.  We
achieved this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
handle_mm_fault() the second time.  This was majorly used to avoid
unexpected starvation of the system by looping over forever to handle
the page fault on a single page.  However that should hardly happen,
and after all for each code path to return a VM_FAULT_RETRY we'll
first wait for a condition (during which time we should possibly yield
the cpu) to happen before VM_FAULT_RETRY is really returned.

This patch removes the restriction by keeping the
FAULT_FLAG_ALLOW_RETRY flag when we receive VM_FAULT_RETRY.  It means
that the page fault handler now can retry the page fault for multiple
times if necessary without the need to generate another page fault
event.  Meanwhile we still keep the FAULT_FLAG_TRIED flag so page
fault handler can still identify whether a page fault is the first
attempt or not.

Then we'll have these combinations of fault flags (only considering
ALLOW_RETRY flag and TRIED flag):

  - ALLOW_RETRY and !TRIED:  this means the page fault allows to
                             retry, and this is the first try

  - ALLOW_RETRY and TRIED:   this means the page fault allows to
                             retry, and this is not the first try

  - !ALLOW_RETRY and !TRIED: this means the page fault does not allow
                             to retry at all

  - !ALLOW_RETRY and TRIED:  this is forbidden and should never be used

In existing code we have multiple places that has taken special care
of the first condition above by checking against (fault_flags &
FAULT_FLAG_ALLOW_RETRY).  This patch introduces a simple helper to
detect the first retry of a page fault by checking against
both (fault_flags & FAULT_FLAG_ALLOW_RETRY) and !(fault_flag &
FAULT_FLAG_TRIED) because now even the 2nd try will have the
ALLOW_RETRY set, then use that helper in all existing special paths.
One example is in __lock_page_or_retry(), now we'll drop the mmap_sem
only in the first attempt of page fault and we'll keep it in follow up
retries, so old locking behavior will be retained.

This will be a nice enhancement for current code [2] at the same time
a supporting material for the future userfaultfd-writeprotect work,
since in that work there will always be an explicit userfault
writeprotect retry for protected pages, and if that cannot resolve the
page fault (e.g., when userfaultfd-writeprotect is used in conjunction
with swapped pages) then we'll possibly need a 3rd retry of the page
fault.  It might also benefit other potential users who will have
similar requirement like userfault write-protection.

GUP code is not touched yet and will be covered in follow up patch.

Please read the thread below for more information.

[1] https://lkml.org/lkml/2017/11/2/833
[2] https://lkml.org/lkml/2018/12/30/64

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/alpha/mm/fault.c           |  2 +-
 arch/arc/mm/fault.c             |  1 -
 arch/arm/mm/fault.c             |  3 ---
 arch/arm64/mm/fault.c           |  5 -----
 arch/hexagon/mm/vm_fault.c      |  1 -
 arch/ia64/mm/fault.c            |  1 -
 arch/m68k/mm/fault.c            |  3 ---
 arch/microblaze/mm/fault.c      |  1 -
 arch/mips/mm/fault.c            |  1 -
 arch/nds32/mm/fault.c           |  1 -
 arch/nios2/mm/fault.c           |  3 ---
 arch/openrisc/mm/fault.c        |  1 -
 arch/parisc/mm/fault.c          |  4 +---
 arch/powerpc/mm/fault.c         |  6 ------
 arch/riscv/mm/fault.c           |  5 -----
 arch/s390/mm/fault.c            |  5 +----
 arch/sh/mm/fault.c              |  1 -
 arch/sparc/mm/fault_32.c        |  1 -
 arch/sparc/mm/fault_64.c        |  1 -
 arch/um/kernel/trap.c           |  1 -
 arch/unicore32/mm/fault.c       |  4 +---
 arch/x86/mm/fault.c             |  2 --
 arch/xtensa/mm/fault.c          |  1 -
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 ++++++++---
 include/linux/mm.h              | 37 +++++++++++++++++++++++++++++++++
 mm/filemap.c                    |  2 +-
 mm/shmem.c                      |  2 +-
 27 files changed, 52 insertions(+), 55 deletions(-)

diff --git a/arch/alpha/mm/fault.c b/arch/alpha/mm/fault.c
index adf5c46ddb68..cb87e72b0a12 100644
--- a/arch/alpha/mm/fault.c
+++ b/arch/alpha/mm/fault.c
@@ -170,7 +170,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c
index 49b6a6802862..27d220e07fe0 100644
--- a/arch/arc/mm/fault.c
+++ b/arch/arc/mm/fault.c
@@ -159,7 +159,6 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 			}
 
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 				goto retry;
 			}
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 9af92c1c5510..f4393ad0b054 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -340,9 +340,6 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 					regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 40cb184068f2..31e110d85edc 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -514,12 +514,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 				return 0;
 		}
 
-		/*
-		 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk of
-		 * starvation.
-		 */
 		if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
-			mm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			mm_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
diff --git a/arch/hexagon/mm/vm_fault.c b/arch/hexagon/mm/vm_fault.c
index e051564b5e33..b6f25ab4e3fa 100644
--- a/arch/hexagon/mm/vm_fault.c
+++ b/arch/hexagon/mm/vm_fault.c
@@ -103,7 +103,6 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 				goto retry;
 			}
diff --git a/arch/ia64/mm/fault.c b/arch/ia64/mm/fault.c
index 54fc86b4f418..b829c2c6b9b1 100644
--- a/arch/ia64/mm/fault.c
+++ b/arch/ia64/mm/fault.c
@@ -190,7 +190,6 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/m68k/mm/fault.c b/arch/m68k/mm/fault.c
index 6376c079ea54..33dec5498069 100644
--- a/arch/m68k/mm/fault.c
+++ b/arch/m68k/mm/fault.c
@@ -163,9 +163,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/microblaze/mm/fault.c b/arch/microblaze/mm/fault.c
index 03c6a4cdf66c..f7da23b65dda 100644
--- a/arch/microblaze/mm/fault.c
+++ b/arch/microblaze/mm/fault.c
@@ -237,7 +237,6 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
index f0b34674a01f..c7db8f08217b 100644
--- a/arch/mips/mm/fault.c
+++ b/arch/mips/mm/fault.c
@@ -179,7 +179,6 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 			tsk->min_flt++;
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/nds32/mm/fault.c b/arch/nds32/mm/fault.c
index d203d9b0f7a0..41e71384dda6 100644
--- a/arch/nds32/mm/fault.c
+++ b/arch/nds32/mm/fault.c
@@ -243,7 +243,6 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 				      1, regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/nios2/mm/fault.c b/arch/nios2/mm/fault.c
index f9f178484184..07f467577e77 100644
--- a/arch/nios2/mm/fault.c
+++ b/arch/nios2/mm/fault.c
@@ -158,9 +158,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/openrisc/mm/fault.c b/arch/openrisc/mm/fault.c
index 26150520bee4..a86e2c4de376 100644
--- a/arch/openrisc/mm/fault.c
+++ b/arch/openrisc/mm/fault.c
@@ -182,7 +182,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/parisc/mm/fault.c b/arch/parisc/mm/fault.c
index 46a0fcecabe1..996f18143886 100644
--- a/arch/parisc/mm/fault.c
+++ b/arch/parisc/mm/fault.c
@@ -328,14 +328,12 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
-
 			/*
 			 * No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
-
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index bf06a407d1a7..06d650919ba6 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -608,13 +608,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * case.
 	 */
 	if (unlikely(fault & VM_FAULT_RETRY)) {
-		/* We retry only once */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			if (is_user && signal_pending(current))
 				return 0;
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index ddd43d9ae45e..0eb8f977ca0d 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -143,11 +143,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 				      1, regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY);
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 62235ec77b03..9f15f0c72c3e 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -527,10 +527,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 				fault = VM_FAULT_PFAULT;
 				goto out_up;
 			}
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY |
-				   FAULT_FLAG_RETRY_NOWAIT);
+			flags &= ~FAULT_FLAG_RETRY_NOWAIT;
 			flags |= FAULT_FLAG_TRIED;
 			down_read(&mm->mmap_sem);
 			goto retry;
diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c
index be559fcaa3ca..09adeaa0eba1 100644
--- a/arch/sh/mm/fault.c
+++ b/arch/sh/mm/fault.c
@@ -498,7 +498,6 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 				      regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*
diff --git a/arch/sparc/mm/fault_32.c b/arch/sparc/mm/fault_32.c
index 9bcbfcb1b8c3..9cab5710e3cc 100644
--- a/arch/sparc/mm/fault_32.c
+++ b/arch/sparc/mm/fault_32.c
@@ -261,7 +261,6 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/sparc/mm/fault_64.c b/arch/sparc/mm/fault_64.c
index 1c9d51b62222..6ae8c5b9abf0 100644
--- a/arch/sparc/mm/fault_64.c
+++ b/arch/sparc/mm/fault_64.c
@@ -460,7 +460,6 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 3eaf37a5539f..232e19036cfc 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -98,7 +98,6 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 
 				goto retry;
diff --git a/arch/unicore32/mm/fault.c b/arch/unicore32/mm/fault.c
index 185c9b1a7a55..a2b18728dc04 100644
--- a/arch/unicore32/mm/fault.c
+++ b/arch/unicore32/mm/fault.c
@@ -259,9 +259,7 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index f1b5df6758e5..84134a30ab77 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1463,9 +1463,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 	 * that we made any progress. Handle this case first.
 	 */
 	if (unlikely(fault & VM_FAULT_RETRY)) {
-		/* Retry at most once */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
 				return;
diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index b9c2ea0de49b..6ab236adda9b 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -129,7 +129,6 @@ void do_page_fault(struct pt_regs *regs)
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 6dacff49c1cc..8f2f9ee6effa 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -61,9 +61,10 @@ static vm_fault_t ttm_bo_vm_fault_idle(struct ttm_buffer_object *bo,
 
 	/*
 	 * If possible, avoid waiting for GPU with mmap_sem
-	 * held.
+	 * held.  We only do this if the fault allows retry and this
+	 * is the first attempt.
 	 */
-	if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(vmf->flags)) {
 		ret = VM_FAULT_RETRY;
 		if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_unlock;
@@ -132,7 +133,12 @@ static vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
 	 * for the buffer to become unreserved.
 	 */
 	if (unlikely(!reservation_object_trylock(bo->resv))) {
-		if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+		/*
+		 * If the fault allows retry and this is the first
+		 * fault attempt, we try to release the mmap_sem
+		 * before waiting
+		 */
+		if (fault_flag_allow_retry_first(vmf->flags)) {
 			if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
 				ttm_bo_get(bo);
 				up_read(&vmf->vma->vm_mm->mmap_sem);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 845a4f5153dd..d0f67cbcd780 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -396,6 +396,25 @@ extern pgprot_t protection_map[16];
  * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
  * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
  * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ *
+ * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
+ * whether we would allow page faults to retry by specifying these two
+ * fault flags correctly.  Currently there can be three legal combinations:
+ *
+ * (a) ALLOW_RETRY and !TRIED:  this means the page fault allows retry, and
+ *                              this is the first try
+ *
+ * (b) ALLOW_RETRY and TRIED:   this means the page fault allows retry, and
+ *                              we've already tried at least once
+ *
+ * (c) !ALLOW_RETRY and !TRIED: this means the page fault does not allow retry
+ *
+ * The unlisted combination (!ALLOW_RETRY && TRIED) is illegal and should never
+ * be used.  Note that page faults can be allowed to retry for multiple times,
+ * in which case we'll have an initial fault with flags (a) then later on
+ * continuous faults with flags (b).  We should always try to detect pending
+ * signals before a retry to make sure the continuous page faults can still be
+ * interrupted if necessary.
  */
 #define FAULT_FLAG_WRITE			0x01
 #define FAULT_FLAG_MKWRITE			0x02
@@ -416,6 +435,24 @@ extern pgprot_t protection_map[16];
 			     FAULT_FLAG_KILLABLE | \
 			     FAULT_FLAG_INTERRUPTIBLE)
 
+/**
+ * fault_flag_allow_retry_first - check ALLOW_RETRY the first time
+ *
+ * This is mostly used for places where we want to try to avoid taking
+ * the mmap_sem for too long a time when waiting for another condition
+ * to change, in which case we can try to be polite to release the
+ * mmap_sem in the first round to avoid potential starvation of other
+ * processes that would also want the mmap_sem.
+ *
+ * Return: true if the page fault allows retry and this is the first
+ * attempt of the fault handling; false otherwise.
+ */
+static inline bool fault_flag_allow_retry_first(unsigned int flags)
+{
+	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
+	    (!(flags & FAULT_FLAG_TRIED));
+}
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \
diff --git a/mm/filemap.c b/mm/filemap.c
index df2006ba0cfa..83fdf429f795 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1381,7 +1381,7 @@ EXPORT_SYMBOL_GPL(__lock_page_killable);
 int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 			 unsigned int flags)
 {
-	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(flags)) {
 		/*
 		 * CAUTION! In this case, mmap_sem is not released
 		 * even though return 0.
diff --git a/mm/shmem.c b/mm/shmem.c
index 1bb3b8dc8bb2..ef3a19c83927 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2009,7 +2009,7 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
 			DEFINE_WAIT_FUNC(shmem_fault_wait, synchronous_wake_function);
 
 			ret = VM_FAULT_NOPAGE;
-			if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
+			if (fault_flag_allow_retry_first(vmf->flags) &&
 			   !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
 				/* It's polite to up mmap_sem if we can */
 				up_read(&vma->vm_mm->mmap_sem);
-- 
2.21.0

[PATCH 7/7] mm/gup: Allow VM_FAULT_RETRY for multiple times

[Peter Xu]

This is the gup counterpart of the change that allows the VM_FAULT_RETRY
to happen for more than once.

Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c     | 17 +++++++++++++----
 mm/hugetlb.c |  6 ++++--
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 58d282115d9b..ac8d5b73c212 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -647,7 +647,10 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
-		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
+		/*
+		 * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
+		 * can co-exist
+		 */
 		fault_flags |= FAULT_FLAG_TRIED;
 	}
 
@@ -1062,17 +1065,23 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		if (likely(pages))
 			pages += ret;
 		start += ret << PAGE_SHIFT;
+		lock_dropped = true;
 
+retry:
 		/*
 		 * Repeat on the address that fired VM_FAULT_RETRY
-		 * without FAULT_FLAG_ALLOW_RETRY but with
+		 * with both FAULT_FLAG_ALLOW_RETRY and
 		 * FAULT_FLAG_TRIED.
 		 */
 		*locked = 1;
-		lock_dropped = true;
 		down_read(&mm->mmap_sem);
 		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
-				       pages, NULL, NULL);
+				       pages, NULL, locked);
+		if (!*locked) {
+			/* Continue to retry until we succeeded */
+			BUG_ON(ret != 0);
+			goto retry;
+		}
 		if (ret != 1) {
 			BUG_ON(ret > 1);
 			if (!pages_done)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ba179c2fa8fb..d9c739f9a28e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4317,8 +4317,10 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_RETRY_NOWAIT;
 			if (flags & FOLL_TRIED) {
-				VM_WARN_ON_ONCE(fault_flags &
-						FAULT_FLAG_ALLOW_RETRY);
+				/*
+				 * Note: FAULT_FLAG_ALLOW_RETRY and
+				 * FAULT_FLAG_TRIED can co-exist
+				 */
 				fault_flags |= FAULT_FLAG_TRIED;
 			}
 			ret = hugetlb_fault(mm, vma, vaddr, fault_flags);
-- 
2.21.0