[PATCH v2 2/2] mm: don't allocate a page table under RCU in do_set_pmd()

["David Hildenbrand (Arm)" <david@kernel.org>]

do_fault_around() skips PTE table preallocation when the PMD already points
to a PTE table. A concurrent collapse can then retract that table and
clear the PMD before filemap_map_pages() locks the new PMD-sized folio.

filemap_map_pmd() can then call do_set_pmd() with an empty PMD and no
preallocated table. On PowerPC hash MMU, do_set_pmd() then tries
allocating a PTE table to deposit. But filemap_map_pmd() calls
do_set_pmd() under RCU, so allocating from that context is not good.

This race became possible when retract_page_tables() stopped taking the
mmap and VMA locks. Previously, those locks excluded retraction while
the fault was in progress.

Add a may_alloc argument to do_set_pmd() and pass false from
filemap_map_pmd(), so we can fail with VM_FAULT_FALLBACK instead of
trying to allocate.

This issue should be fairly hard to hit in practice.

Fixes: 1d65b771bc08 ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
Cc: stable@vger.kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 include/linux/mm.h |  3 ++-
 mm/filemap.c       |  2 +-
 mm/khugepaged.c    |  2 +-
 mm/memory.c        | 10 +++++++---
 4 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a2dd76cc5bbfe..bd144fec5f5da 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2156,7 +2156,8 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
 	return pte;
 }
 
-vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page);
+vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page,
+		bool may_alloc);
 void set_pte_range(struct vm_fault *vmf, struct folio *folio,
 		struct page *page, unsigned int nr, unsigned long addr);
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 86297841f9955..9871b4f32a916 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3745,7 +3745,7 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
 
 	if (pmd_none(*vmf->pmd) && folio_test_pmd_mappable(folio)) {
 		struct page *page = folio_file_page(folio, start);
-		vm_fault_t ret = do_set_pmd(vmf, folio, page);
+		vm_fault_t ret = do_set_pmd(vmf, folio, page, /* may_alloc= */ false);
 		if (!ret) {
 			/* The page is mapped successfully, reference consumed. */
 			folio_unlock(folio);
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8d9f05a39afb3..5836e08abc984 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1826,7 +1826,7 @@ static enum scan_result set_huge_pmd(struct vm_area_struct *vma, unsigned long a
 	}
 
 	vmf.pmd = pmdp;
-	if (do_set_pmd(&vmf, folio, page))
+	if (do_set_pmd(&vmf, folio, page, /* may_alloc= */ true))
 		return SCAN_FAIL;
 
 	folio_get(folio);
diff --git a/mm/memory.c b/mm/memory.c
index 421064aef3cf0..6eeed43b05a19 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5682,7 +5682,8 @@ static void deposit_prealloc_pte(struct vm_fault *vmf)
 	vmf->prealloc_pte = NULL;
 }
 
-vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page)
+vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page,
+		bool may_alloc)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	bool write = vmf->flags & FAULT_FLAG_WRITE;
@@ -5723,6 +5724,8 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
 	 * related to pte entry. Use the preallocated table for that.
 	 */
 	if (arch_needs_pgtable_deposit() && !vmf->prealloc_pte) {
+		if (!may_alloc)
+			return VM_FAULT_FALLBACK;
 		vmf->prealloc_pte = pte_alloc_one(vma->vm_mm);
 		if (!vmf->prealloc_pte)
 			return VM_FAULT_OOM;
@@ -5764,7 +5767,8 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *pa
 	return ret;
 }
 #else
-vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page)
+vm_fault_t do_set_pmd(struct vm_fault *vmf, struct folio *folio, struct page *page,
+		bool may_alloc)
 {
 	return VM_FAULT_FALLBACK;
 }
@@ -5888,7 +5892,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 
 	if (pmd_none(*vmf->pmd)) {
 		if (!needs_fallback && folio_test_pmd_mappable(folio)) {
-			ret = do_set_pmd(vmf, folio, page);
+			ret = do_set_pmd(vmf, folio, page, /* may_alloc= */ true);
 			if (ret != VM_FAULT_FALLBACK)
 				return ret;
 		}

-- 
2.43.0