[PATCH v3 1/3] seqlock: add raw_seqcount_try_begin

[PATCH v3 1/3] seqlock: add raw_seqcount_try_begin

[Suren Baghdasaryan]

Add raw_seqcount_try_begin() to opens a read critical section of the given
seqcount_t if the counter is even. This enables eliding the critical
section entirely if the counter is odd, instead of doing the speculation
knowing it will fail.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
Applies over Linus' ToT

Changes since v2 [1]
- Added SOB, per David Hildenbrand
- Added SOB, per Liam Howlett

[1] https://lore.kernel.org/all/20241121162826.987947-1-surenb@google.com/

 include/linux/seqlock.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5298765d6ca4..22c2c48b4265 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -318,6 +318,28 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 	__seq;								\
 })
 
+/**
+ * raw_seqcount_try_begin() - begin a seqcount_t read critical section
+ *                            w/o lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
+ *
+ * Similar to raw_seqcount_begin(), except it enables eliding the critical
+ * section entirely if odd, instead of doing the speculation knowing it will
+ * fail.
+ *
+ * Useful when counter stabilization is more or less equivalent to taking
+ * the lock and there is a slowpath that does that.
+ *
+ * If true, start will be set to the (even) sequence count read.
+ *
+ * Return: true when a read critical section is started.
+ */
+#define raw_seqcount_try_begin(s, start)				\
+({									\
+	start = raw_read_seqcount(s);					\
+	!(start & 1);							\
+})
+
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization

base-commit: 28eb75e178d389d325f1666e422bc13bbbb9804c
-- 
2.47.0.371.ga323438b13-goog

[PATCH v3 2/3] mm: convert mm_lock_seq to a proper seqcount

[Suren Baghdasaryan]

Convert mm_lock_seq to be seqcount_t and change all mmap_write_lock
variants to increment it, in-line with the usual seqcount usage pattern.
This lets us check whether the mmap_lock is write-locked by checking
mm_lock_seq.sequence counter (odd=locked, even=unlocked). This will be
used when implementing mmap_lock speculation functions.
As a result vm_lock_seq is also change to be unsigned to match the type
of mm_lock_seq.sequence.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
Changes since v2 [1]
- Moved ASSERT_EXCLUSIVE_WRITER() into mm_lock_seqcount_end to fix
!CONFIG_PER_VMA_LOCK, per kernel test bot
- Added SOB, per Liam Howlett

[1] https://lore.kernel.org/all/20241121162826.987947-2-surenb@google.com/

 include/linux/mm.h               | 12 +++----
 include/linux/mm_types.h         |  7 ++--
 include/linux/mmap_lock.h        | 55 +++++++++++++++++++++-----------
 kernel/fork.c                    |  5 +--
 mm/init-mm.c                     |  2 +-
 tools/testing/vma/vma.c          |  4 +--
 tools/testing/vma/vma_internal.h |  4 +--
 7 files changed, 53 insertions(+), 36 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 673771f34674..a553131644e7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -710,7 +710,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * we don't rely on for anything - the mm_lock_seq read against which we
 	 * need ordering is below.
 	 */
-	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq))
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
 		return false;
 
 	if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
@@ -727,7 +727,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(vma->vm_lock_seq == smp_load_acquire(&vma->vm_mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
 		up_read(&vma->vm_lock->lock);
 		return false;
 	}
@@ -742,7 +742,7 @@ static inline void vma_end_read(struct vm_area_struct *vma)
 }
 
 /* WARNING! Can only be used if mmap_lock is expected to be write-locked */
-static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
+static bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
 {
 	mmap_assert_write_locked(vma->vm_mm);
 
@@ -750,7 +750,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
 	 * current task is holding mmap_write_lock, both vma->vm_lock_seq and
 	 * mm->mm_lock_seq can't be concurrently modified.
 	 */
-	*mm_lock_seq = vma->vm_mm->mm_lock_seq;
+	*mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
 	return (vma->vm_lock_seq == *mm_lock_seq);
 }
 
@@ -761,7 +761,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
  */
 static inline void vma_start_write(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	if (__is_vma_write_locked(vma, &mm_lock_seq))
 		return;
@@ -779,7 +779,7 @@ static inline void vma_start_write(struct vm_area_struct *vma)
 
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index e85beea1206e..4e06bde7ec36 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -697,7 +697,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif
@@ -891,6 +891,9 @@ struct mm_struct {
 		 * Roughly speaking, incrementing the sequence number is
 		 * equivalent to releasing locks on VMAs; reading the sequence
 		 * number can be part of taking a read lock on a VMA.
+		 * Incremented every time mmap_lock is write-locked/unlocked.
+		 * Initialized to 0, therefore odd values indicate mmap_lock
+		 * is write-locked and even values that it's released.
 		 *
 		 * Can be modified under write mmap_lock using RELEASE
 		 * semantics.
@@ -899,7 +902,7 @@ struct mm_struct {
 		 * Can be read with ACQUIRE semantics if not holding write
 		 * mmap_lock.
 		 */
-		int mm_lock_seq;
+		seqcount_t mm_lock_seq;
 #endif
 
 
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index de9dc20b01ba..9715326f5a85 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,39 +71,39 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
-/*
- * Drop all currently-held per-VMA locks.
- * This is called from the mmap_lock implementation directly before releasing
- * a write-locked mmap_lock (or downgrading it to read-locked).
- * This should normally NOT be called manually from other places.
- * If you want to call this manually anyway, keep in mind that this will release
- * *all* VMA write locks, including ones from further up the stack.
- */
-static inline void vma_end_write_all(struct mm_struct *mm)
+static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
-	mmap_assert_write_locked(mm);
-	/*
-	 * Nobody can concurrently modify mm->mm_lock_seq due to exclusive
-	 * mmap_lock being held.
-	 * We need RELEASE semantics here to ensure that preceding stores into
-	 * the VMA take effect before we unlock it with this store.
-	 * Pairs with ACQUIRE semantics in vma_start_read().
-	 */
-	smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
+	seqcount_init(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm)
+{
+	do_raw_write_seqcount_begin(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_end(struct mm_struct *mm)
+{
+	ASSERT_EXCLUSIVE_WRITER(mm->mm_lock_seq);
+	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
+
 #else
-static inline void vma_end_write_all(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
 #endif
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
 	init_rwsem(&mm->mmap_lock);
+	mm_lock_seqcount_init(mm);
 }
 
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -111,6 +111,7 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write_nested(&mm->mmap_lock, subclass);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -120,10 +121,26 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
 
 	__mmap_lock_trace_start_locking(mm, true);
 	ret = down_write_killable(&mm->mmap_lock);
+	if (!ret)
+		mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, ret == 0);
 	return ret;
 }
 
+/*
+ * Drop all currently-held per-VMA locks.
+ * This is called from the mmap_lock implementation directly before releasing
+ * a write-locked mmap_lock (or downgrading it to read-locked).
+ * This should normally NOT be called manually from other places.
+ * If you want to call this manually anyway, keep in mind that this will release
+ * *all* VMA write locks, including ones from further up the stack.
+ */
+static inline void vma_end_write_all(struct mm_struct *mm)
+{
+	mmap_assert_write_locked(mm);
+	mm_lock_seqcount_end(mm);
+}
+
 static inline void mmap_write_unlock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_released(mm, true);
diff --git a/kernel/fork.c b/kernel/fork.c
index e58d27c05788..8cd36645b9fc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -449,7 +449,7 @@ static bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }
@@ -1262,9 +1262,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	seqcount_init(&mm->write_protect_seq);
 	mmap_init_lock(mm);
 	INIT_LIST_HEAD(&mm->mmlist);
-#ifdef CONFIG_PER_VMA_LOCK
-	mm->mm_lock_seq = 0;
-#endif
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;
 	mm->locked_vm = 0;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index 24c809379274..6af3ad675930 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -40,7 +40,7 @@ struct mm_struct init_mm = {
 	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 #ifdef CONFIG_PER_VMA_LOCK
-	.mm_lock_seq	= 0,
+	.mm_lock_seq	= SEQCNT_ZERO(init_mm.mm_lock_seq),
 #endif
 	.user_ns	= &init_user_ns,
 	.cpu_bitmap	= CPU_BITS_NONE,
diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
index b33b47342d41..9074aaced9c5 100644
--- a/tools/testing/vma/vma.c
+++ b/tools/testing/vma/vma.c
@@ -87,7 +87,7 @@ static struct vm_area_struct *alloc_and_link_vma(struct mm_struct *mm,
 	 * begun. Linking to the tree will have caused this to be incremented,
 	 * which means we will get a false positive otherwise.
 	 */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return vma;
 }
@@ -212,7 +212,7 @@ static bool vma_write_started(struct vm_area_struct *vma)
 	int seq = vma->vm_lock_seq;
 
 	/* We reset after each check. */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	/* The vma_start_write() stub simply increments this value. */
 	return seq > -1;
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index c5b9da034511..4007ec580f85 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -231,7 +231,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	struct vma_lock *vm_lock;
 #endif
 
@@ -406,7 +406,7 @@ static inline bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }
-- 
2.47.0.371.ga323438b13-goog

[tip: perf/core] mm: convert mm_lock_seq to a proper seqcount

[tip-bot2 for Suren Baghdasaryan]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     eb449bd96954b1c1e491d19066cfd2a010f0aa47
Gitweb:        https://git.kernel.org/tip/eb449bd96954b1c1e491d19066cfd2a010f0aa47
Author:        Suren Baghdasaryan <surenb@google.com>
AuthorDate:    Fri, 22 Nov 2024 09:44:15 -08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 02 Dec 2024 12:01:38 +01:00

mm: convert mm_lock_seq to a proper seqcount

Convert mm_lock_seq to be seqcount_t and change all mmap_write_lock
variants to increment it, in-line with the usual seqcount usage pattern.
This lets us check whether the mmap_lock is write-locked by checking
mm_lock_seq.sequence counter (odd=locked, even=unlocked). This will be
used when implementing mmap_lock speculation functions.
As a result vm_lock_seq is also change to be unsigned to match the type
of mm_lock_seq.sequence.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-2-surenb@google.com
---
 include/linux/mm.h               | 12 +++----
 include/linux/mm_types.h         |  7 ++--
 include/linux/mmap_lock.h        | 55 ++++++++++++++++++++-----------
 kernel/fork.c                    |  5 +---
 mm/init-mm.c                     |  2 +-
 tools/testing/vma/vma.c          |  4 +-
 tools/testing/vma/vma_internal.h |  4 +-
 7 files changed, 53 insertions(+), 36 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c39c494..ca59d16 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -710,7 +710,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * we don't rely on for anything - the mm_lock_seq read against which we
 	 * need ordering is below.
 	 */
-	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq))
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
 		return false;
 
 	if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
@@ -727,7 +727,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
 	 * after it has been unlocked.
 	 * This pairs with RELEASE semantics in vma_end_write_all().
 	 */
-	if (unlikely(vma->vm_lock_seq == smp_load_acquire(&vma->vm_mm->mm_lock_seq))) {
+	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
 		up_read(&vma->vm_lock->lock);
 		return false;
 	}
@@ -742,7 +742,7 @@ static inline void vma_end_read(struct vm_area_struct *vma)
 }
 
 /* WARNING! Can only be used if mmap_lock is expected to be write-locked */
-static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
+static bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
 {
 	mmap_assert_write_locked(vma->vm_mm);
 
@@ -750,7 +750,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
 	 * current task is holding mmap_write_lock, both vma->vm_lock_seq and
 	 * mm->mm_lock_seq can't be concurrently modified.
 	 */
-	*mm_lock_seq = vma->vm_mm->mm_lock_seq;
+	*mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
 	return (vma->vm_lock_seq == *mm_lock_seq);
 }
 
@@ -761,7 +761,7 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
  */
 static inline void vma_start_write(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	if (__is_vma_write_locked(vma, &mm_lock_seq))
 		return;
@@ -779,7 +779,7 @@ static inline void vma_start_write(struct vm_area_struct *vma)
 
 static inline void vma_assert_write_locked(struct vm_area_struct *vma)
 {
-	int mm_lock_seq;
+	unsigned int mm_lock_seq;
 
 	VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
 }
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 7361a8f..97e2f4f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -697,7 +697,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	/* Unstable RCU readers are allowed to read this. */
 	struct vma_lock *vm_lock;
 #endif
@@ -891,6 +891,9 @@ struct mm_struct {
 		 * Roughly speaking, incrementing the sequence number is
 		 * equivalent to releasing locks on VMAs; reading the sequence
 		 * number can be part of taking a read lock on a VMA.
+		 * Incremented every time mmap_lock is write-locked/unlocked.
+		 * Initialized to 0, therefore odd values indicate mmap_lock
+		 * is write-locked and even values that it's released.
 		 *
 		 * Can be modified under write mmap_lock using RELEASE
 		 * semantics.
@@ -899,7 +902,7 @@ struct mm_struct {
 		 * Can be read with ACQUIRE semantics if not holding write
 		 * mmap_lock.
 		 */
-		int mm_lock_seq;
+		seqcount_t mm_lock_seq;
 #endif
 
 
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index de9dc20..9715326 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,39 +71,39 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
-/*
- * Drop all currently-held per-VMA locks.
- * This is called from the mmap_lock implementation directly before releasing
- * a write-locked mmap_lock (or downgrading it to read-locked).
- * This should normally NOT be called manually from other places.
- * If you want to call this manually anyway, keep in mind that this will release
- * *all* VMA write locks, including ones from further up the stack.
- */
-static inline void vma_end_write_all(struct mm_struct *mm)
+static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
-	mmap_assert_write_locked(mm);
-	/*
-	 * Nobody can concurrently modify mm->mm_lock_seq due to exclusive
-	 * mmap_lock being held.
-	 * We need RELEASE semantics here to ensure that preceding stores into
-	 * the VMA take effect before we unlock it with this store.
-	 * Pairs with ACQUIRE semantics in vma_start_read().
-	 */
-	smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
+	seqcount_init(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm)
+{
+	do_raw_write_seqcount_begin(&mm->mm_lock_seq);
+}
+
+static inline void mm_lock_seqcount_end(struct mm_struct *mm)
+{
+	ASSERT_EXCLUSIVE_WRITER(mm->mm_lock_seq);
+	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
+
 #else
-static inline void vma_end_write_all(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
+static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
 #endif
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
 	init_rwsem(&mm->mmap_lock);
+	mm_lock_seqcount_init(mm);
 }
 
 static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -111,6 +111,7 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write_nested(&mm->mmap_lock, subclass);
+	mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -120,10 +121,26 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
 
 	__mmap_lock_trace_start_locking(mm, true);
 	ret = down_write_killable(&mm->mmap_lock);
+	if (!ret)
+		mm_lock_seqcount_begin(mm);
 	__mmap_lock_trace_acquire_returned(mm, true, ret == 0);
 	return ret;
 }
 
+/*
+ * Drop all currently-held per-VMA locks.
+ * This is called from the mmap_lock implementation directly before releasing
+ * a write-locked mmap_lock (or downgrading it to read-locked).
+ * This should normally NOT be called manually from other places.
+ * If you want to call this manually anyway, keep in mind that this will release
+ * *all* VMA write locks, including ones from further up the stack.
+ */
+static inline void vma_end_write_all(struct mm_struct *mm)
+{
+	mmap_assert_write_locked(mm);
+	mm_lock_seqcount_end(mm);
+}
+
 static inline void mmap_write_unlock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_released(mm, true);
diff --git a/kernel/fork.c b/kernel/fork.c
index 1450b46..8dc670f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -448,7 +448,7 @@ static bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }
@@ -1267,9 +1267,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	seqcount_init(&mm->write_protect_seq);
 	mmap_init_lock(mm);
 	INIT_LIST_HEAD(&mm->mmlist);
-#ifdef CONFIG_PER_VMA_LOCK
-	mm->mm_lock_seq = 0;
-#endif
 	mm_pgtables_bytes_init(mm);
 	mm->map_count = 0;
 	mm->locked_vm = 0;
diff --git a/mm/init-mm.c b/mm/init-mm.c
index 24c8093..6af3ad6 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -40,7 +40,7 @@ struct mm_struct init_mm = {
 	.arg_lock	=  __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
 	.mmlist		= LIST_HEAD_INIT(init_mm.mmlist),
 #ifdef CONFIG_PER_VMA_LOCK
-	.mm_lock_seq	= 0,
+	.mm_lock_seq	= SEQCNT_ZERO(init_mm.mm_lock_seq),
 #endif
 	.user_ns	= &init_user_ns,
 	.cpu_bitmap	= CPU_BITS_NONE,
diff --git a/tools/testing/vma/vma.c b/tools/testing/vma/vma.c
index 8fab5e1..9bcf173 100644
--- a/tools/testing/vma/vma.c
+++ b/tools/testing/vma/vma.c
@@ -89,7 +89,7 @@ static struct vm_area_struct *alloc_and_link_vma(struct mm_struct *mm,
 	 * begun. Linking to the tree will have caused this to be incremented,
 	 * which means we will get a false positive otherwise.
 	 */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return vma;
 }
@@ -214,7 +214,7 @@ static bool vma_write_started(struct vm_area_struct *vma)
 	int seq = vma->vm_lock_seq;
 
 	/* We reset after each check. */
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	/* The vma_start_write() stub simply increments this value. */
 	return seq > -1;
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index e76ff57..1d9fc97 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -241,7 +241,7 @@ struct vm_area_struct {
 	 * counter reuse can only lead to occasional unnecessary use of the
 	 * slowpath.
 	 */
-	int vm_lock_seq;
+	unsigned int vm_lock_seq;
 	struct vma_lock *vm_lock;
 #endif
 
@@ -416,7 +416,7 @@ static inline bool vma_lock_alloc(struct vm_area_struct *vma)
 		return false;
 
 	init_rwsem(&vma->vm_lock->lock);
-	vma->vm_lock_seq = -1;
+	vma->vm_lock_seq = UINT_MAX;
 
 	return true;
 }

[PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Suren Baghdasaryan]

Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
Changes since v2 [1]
- Added SOB, per Liam Howlett

[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/

 include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 9715326f5a85..8ac3041df053 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
 	seqcount_init(&mm->mm_lock_seq);
@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
 
-#else
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	/*
+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
+	 * unlocked is more or less equivalent with taking it ourselves, don't
+	 * bother with the speculative path if mmap_lock is already write-locked
+	 * and take the slow path, which takes the lock.
+	 */
+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
-#endif
+
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	return false;
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return true;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
-- 
2.47.0.371.ga323438b13-goog

Re: [PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Wei Yang]

On Fri, Nov 22, 2024 at 09:44:16AM -0800, Suren Baghdasaryan wrote:
>Add helper functions to speculatively perform operations without
>read-locking mmap_lock, expecting that mmap_lock will not be
>write-locked and mm is not modified from under us.
>
>Suggested-by: Peter Zijlstra <peterz@infradead.org>
>Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
>---
>Changes since v2 [1]
>- Added SOB, per Liam Howlett
>
>[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/
>
> include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> 1 file changed, 31 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
>index 9715326f5a85..8ac3041df053 100644
>--- a/include/linux/mmap_lock.h
>+++ b/include/linux/mmap_lock.h
>@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> }
> 
> #ifdef CONFIG_PER_VMA_LOCK
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> {
> 	seqcount_init(&mm->mm_lock_seq);
>@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
> }
> 
>-#else
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	/*
>+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
>+	 * unlocked is more or less equivalent with taking it ourselves, don't
>+	 * bother with the speculative path if mmap_lock is already write-locked
>+	 * and take the slow path, which takes the lock.
>+	 */
>+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return do_read_seqcount_retry(&mm->mm_lock_seq, seq);

Just curious why we don't use read_seqcount_retry().

Looks this is the only user outside seqlock.h.

>+}
>+
>+#else /* CONFIG_PER_VMA_LOCK */
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
>-#endif
>+
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	return false;
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return true;
>+}
>+
>+#endif /* CONFIG_PER_VMA_LOCK */
> 
> static inline void mmap_init_lock(struct mm_struct *mm)
> {
>-- 
>2.47.0.371.ga323438b13-goog

-- 
Wei Yang
Help you, Help me

Re: [PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Suren Baghdasaryan]

On Sun, Nov 24, 2024 at 4:58 PM Wei Yang <richard.weiyang@gmail.com> wrote:
>
> On Fri, Nov 22, 2024 at 09:44:16AM -0800, Suren Baghdasaryan wrote:
> >Add helper functions to speculatively perform operations without
> >read-locking mmap_lock, expecting that mmap_lock will not be
> >write-locked and mm is not modified from under us.
> >
> >Suggested-by: Peter Zijlstra <peterz@infradead.org>
> >Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> >Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> >---
> >Changes since v2 [1]
> >- Added SOB, per Liam Howlett
> >
> >[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@google.com/
> >
> > include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> > 1 file changed, 31 insertions(+), 2 deletions(-)
> >
> >diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> >index 9715326f5a85..8ac3041df053 100644
> >--- a/include/linux/mmap_lock.h
> >+++ b/include/linux/mmap_lock.h
> >@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> > }
> >
> > #ifdef CONFIG_PER_VMA_LOCK
> >+
> > static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> > {
> >       seqcount_init(&mm->mm_lock_seq);
> >@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> >       do_raw_write_seqcount_end(&mm->mm_lock_seq);
> > }
> >
> >-#else
> >+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> >+{
> >+      /*
> >+       * Since mmap_lock is a sleeping lock, and waiting for it to become
> >+       * unlocked is more or less equivalent with taking it ourselves, don't
> >+       * bother with the speculative path if mmap_lock is already write-locked
> >+       * and take the slow path, which takes the lock.
> >+       */
> >+      return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
> >+}
> >+
> >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> >+{
> >+      return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
>
> Just curious why we don't use read_seqcount_retry().
>
> Looks this is the only user outside seqlock.h.

Ah, good eye! read_seqcount_retry() would be better.

Peter, do you want me to post a new patchset or you can patch it when
picking it up?

>
> >+}
> >+
> >+#else /* CONFIG_PER_VMA_LOCK */
> >+
> > static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> > static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> > static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
> >-#endif
> >+
> >+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
> >+{
> >+      return false;
> >+}
> >+
> >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> >+{
> >+      return true;
> >+}
> >+
> >+#endif /* CONFIG_PER_VMA_LOCK */
> >
> > static inline void mmap_init_lock(struct mm_struct *mm)
> > {
> >--
> >2.47.0.371.ga323438b13-goog
>
> --
> Wei Yang
> Help you, Help me

Re: [PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Peter Zijlstra]

On Mon, Nov 25, 2024 at 08:18:38AM -0800, Suren Baghdasaryan wrote:

> > >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> > >+{
> > >+      return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> >
> > Just curious why we don't use read_seqcount_retry().
> >
> > Looks this is the only user outside seqlock.h.
> 
> Ah, good eye! read_seqcount_retry() would be better.
> 
> Peter, do you want me to post a new patchset or you can patch it when
> picking it up?

Fixed up my local copy, thanks!

Re: [PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Andrew Morton]

On Mon, 25 Nov 2024 17:53:57 +0100 Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Nov 25, 2024 at 08:18:38AM -0800, Suren Baghdasaryan wrote:
> 
> > > >+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
> > > >+{
> > > >+      return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> > >
> > > Just curious why we don't use read_seqcount_retry().
> > >
> > > Looks this is the only user outside seqlock.h.
> > 
> > Ah, good eye! read_seqcount_retry() would be better.
> > 
> > Peter, do you want me to post a new patchset or you can patch it when
> > picking it up?
> 
> Fixed up my local copy, thanks!

I take this to mean that you'll be merging these.  I don't see this
series in -next and I'm sitting on the v1 series, which I shall now
drop.

[tip: perf/core] mm: introduce mmap_lock_speculate_{try_begin|retry}

[tip-bot2 for Suren Baghdasaryan]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     03a001b156d2da186a5618de242750d06bf81e2d
Gitweb:        https://git.kernel.org/tip/03a001b156d2da186a5618de242750d06bf81e2d
Author:        Suren Baghdasaryan <surenb@google.com>
AuthorDate:    Fri, 22 Nov 2024 09:44:16 -08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 02 Dec 2024 12:01:38 +01:00

mm: introduce mmap_lock_speculate_{try_begin|retry}

Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-3-surenb@google.com
---
 include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 9715326..45a21fa 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
 	seqcount_init(&mm->mm_lock_seq);
@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
 
-#else
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	/*
+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
+	 * unlocked is more or less equivalent with taking it ourselves, don't
+	 * bother with the speculative path if mmap_lock is already write-locked
+	 * and take the slow path, which takes the lock.
+	 */
+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
-#endif
+
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	return false;
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+	return true;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {

[tip: perf/core] seqlock: add raw_seqcount_try_begin

[tip-bot2 for Suren Baghdasaryan]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     96450ead16527cbef559b5bd046182e731228f95
Gitweb:        https://git.kernel.org/tip/96450ead16527cbef559b5bd046182e731228f95
Author:        Suren Baghdasaryan <surenb@google.com>
AuthorDate:    Fri, 22 Nov 2024 09:44:14 -08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 02 Dec 2024 12:01:37 +01:00

seqlock: add raw_seqcount_try_begin

Add raw_seqcount_try_begin() to opens a read critical section of the given
seqcount_t if the counter is even. This enables eliding the critical
section entirely if the counter is odd, instead of doing the speculation
knowing it will fail.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Link: https://lkml.kernel.org/r/20241122174416.1367052-1-surenb@google.com
---
 include/linux/seqlock.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5298765..22c2c48 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -319,6 +319,28 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 })
 
 /**
+ * raw_seqcount_try_begin() - begin a seqcount_t read critical section
+ *                            w/o lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
+ *
+ * Similar to raw_seqcount_begin(), except it enables eliding the critical
+ * section entirely if odd, instead of doing the speculation knowing it will
+ * fail.
+ *
+ * Useful when counter stabilization is more or less equivalent to taking
+ * the lock and there is a slowpath that does that.
+ *
+ * If true, start will be set to the (even) sequence count read.
+ *
+ * Return: true when a read critical section is started.
+ */
+#define raw_seqcount_try_begin(s, start)				\
+({									\
+	start = raw_read_seqcount(s);					\
+	!(start & 1);							\
+})
+
+/**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization
  * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants