FAILED: patch "[PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start()" failed to apply to 6.12-stable tree

PATCH New [email protected] 2 participants 3 messages Mar 30 – Apr 1

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author maintainer

2026-03-30 09:51 UTC #0 lore raw

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x ffef67b93aa352b34e6aeba3d52c19a63885409a
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026033034-amount-briar-a849@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From ffef67b93aa352b34e6aeba3d52c19a63885409a Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <[email protected]>
Date: Mon, 23 Mar 2026 21:20:18 +0100
Subject: [PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start()

follow_pfnmap_start() suffers from two problems:

(1) We are not re-fetching the pmd/pud after taking the PTL

Therefore, we are not properly stabilizing what the lock actually protects. If there is concurrent zapping, we would indicate to the caller that we found an entry, however, that entry might already have been invalidated, or contain a different PFN after taking the lock.

Properly use pmdp_get() / pudp_get() after taking the lock.

(2) pmd_leaf() / pud_leaf() are not well defined on non-present entries

pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries.

There is no real guarantee that pmd_leaf()/pud_leaf() returns something reasonable on non-present entries. Most architectures indeed either perform a present check or make it work by smart use of flags.

However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(), and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not do that.

Let's check pmd_present()/pud_present() before assuming "the is a present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page table handling code that traverses user page tables does.

Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, (1) is likely more relevant than (2). It is questionable how often (1) would actually trigger, but let's CC stable to be sure.

This was found by code inspection.

Link https://lkml.kernel.org/r/[email protected] open here

Fixes 6da8e9634bb7 ("mm: new follow_pfnmap API")

Signed-off-by David Hildenbrand (Arm) <[email protected]>

Acked-by Mike Rapoport (Microsoft) <[email protected]>

Reviewed-by Lorenzo Stoakes (Oracle) <[email protected]>

Cc Liam Howlett <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Peter Xu <[email protected]>

Cc Suren Baghdasaryan <[email protected]>

Cc Vlastimil Babka <[email protected]>

Cc <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

mm/memory.c +15 -3

index 2f815a34d924..c65e82c86fed 100644
@@ -6815,11 +6815,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pudp = pud_offset(p4dp, address);
 	pud = pudp_get(pudp);
-	if (pud_none(pud))
+	if (!pud_present(pud))
 		goto out;
 	if (pud_leaf(pud)) {
 		lock = pud_lock(mm, pudp);
-		if (!unlikely(pud_leaf(pud))) {
+		pud = pudp_get(pudp);
+
+		if (unlikely(!pud_present(pud))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pud_leaf(pud))) {
 			spin_unlock(lock);
 			goto retry;
 		}

@@ -6831,9 +6836,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pmdp = pmd_offset(pudp, address);
 	pmd = pmdp_get_lockless(pmdp);
+	if (!pmd_present(pmd))
+		goto out;
 	if (pmd_leaf(pmd)) {
 		lock = pmd_lock(mm, pmdp);
-		if (!unlikely(pmd_leaf(pmd))) {
+		pmd = pmdp_get(pmdp);
+
+		if (unlikely(!pmd_present(pmd))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pmd_leaf(pmd))) {
 			spin_unlock(lock);
 			goto retry;
 		}

Sasha Levin

maintainer

2026-04-01 17:06 UTC #1 lore raw

PATCH 1/2 mm: replace READ_ONCE() with standard page table accessors

From: Anshuman Khandual <[email protected]>

[ Upstream commit c0efdb373c3aaacb32db59cadb0710cac13e44ae ]

Replace all READ_ONCE() with a standard page table accessors i.e pxdp_get() that defaults into READ_ONCE() in cases where platform does not override.

Link https://lkml.kernel.org/r/[email protected] open here

Signed-off-by Anshuman Khandual <[email protected]>

Acked-by David Hildenbrand <[email protected]>

Reviewed-by Lance Yang <[email protected]>

Reviewed-by Wei Yang <[email protected]>

Reviewed-by Dev Jain <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

Stable-dep-of: ffef67b93aa3 ("mm/memory: fix PMD/PUD checks in follow_pfnmap_start()")

Signed-off-by Sasha Levin <[email protected]>

---
 mm/gup.c            | 10 +++++-----
 mm/hmm.c            |  2 +-
 mm/memory.c         |  4 ++--
 mm/mprotect.c       |  2 +-
 mm/sparse-vmemmap.c |  2 +-
 mm/vmscan.c         |  2 +-
 6 files changed, 11 insertions(+), 11 deletions(-)

mm/gup.c +5 -5

index d105817a0c9aa..937865ecfae00 100644
@@ -1013,7 +1013,7 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
 	struct mm_struct *mm = vma->vm_mm;
 
 	pudp = pud_offset(p4dp, address);
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (!pud_present(pud))
 		return no_page_table(vma, flags, address);
 	if (pud_leaf(pud)) {

@@ -1038,7 +1038,7 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
 	p4d_t *p4dp, p4d;
 
 	p4dp = p4d_offset(pgdp, address);
-	p4d = READ_ONCE(*p4dp);
+	p4d = p4dp_get(p4dp);
 	BUILD_BUG_ON(p4d_leaf(p4d));
 
 	if (!p4d_present(p4d) || p4d_bad(p4d))

@@ -3301,7 +3301,7 @@ static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
 
 	pudp = pud_offset_lockless(p4dp, p4d, addr);
 	do {
-		pud_t pud = READ_ONCE(*pudp);
+		pud_t pud = pudp_get(pudp);
 
 		next = pud_addr_end(addr, end);
 		if (unlikely(!pud_present(pud)))

@@ -3327,7 +3327,7 @@ static int gup_fast_p4d_range(pgd_t *pgdp, pgd_t pgd, unsigned long addr,
 
 	p4dp = p4d_offset_lockless(pgdp, pgd, addr);
 	do {
-		p4d_t p4d = READ_ONCE(*p4dp);
+		p4d_t p4d = p4dp_get(p4dp);
 
 		next = p4d_addr_end(addr, end);
 		if (!p4d_present(p4d))

@@ -3349,7 +3349,7 @@ static void gup_fast_pgd_range(unsigned long addr, unsigned long end,
 
 	pgdp = pgd_offset(current->mm, addr);
 	do {
-		pgd_t pgd = READ_ONCE(*pgdp);
+		pgd_t pgd = pgdp_get(pgdp);
 
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))

mm/hmm.c +1 -1

index a67776aeb0199..a27866a1d9bd5 100644
@@ -423,7 +423,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 	/* Normally we don't want to split the huge page */
 	walk->action = ACTION_CONTINUE;
 
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (!pud_present(pud)) {
 		spin_unlock(ptl);
 		return hmm_vma_walk_hole(start, end, -1, walk);

mm/memory.c +2 -2

index 090e9c6f99920..d27cd9a7443ce 100644
@@ -6451,12 +6451,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 		goto out;
 
 	p4dp = p4d_offset(pgdp, address);
-	p4d = READ_ONCE(*p4dp);
+	p4d = p4dp_get(p4dp);
 	if (p4d_none(p4d) || unlikely(p4d_bad(p4d)))
 		goto out;
 
 	pudp = pud_offset(p4dp, address);
-	pud = READ_ONCE(*pudp);
+	pud = pudp_get(pudp);
 	if (pud_none(pud))
 		goto out;
 	if (pud_leaf(pud)) {

mm/mprotect.c +1 -1

index 6f450af3252eb..a7c2d7c68a6a5 100644
@@ -447,7 +447,7 @@ static inline long change_pud_range(struct mmu_gather *tlb,
 			break;
 		}
 
-		pud = READ_ONCE(*pudp);
+		pud = pudp_get(pudp);
 		if (pud_none(pud))
 			continue;

mm/sparse-vmemmap.c +1 -1

index c3353cd442a5d..3e88708886e37 100644
@@ -337,7 +337,7 @@ int __meminit vmemmap_populate_hugepages(unsigned long start, unsigned long end,
 			return -ENOMEM;
 
 		pmd = pmd_offset(pud, addr);
-		if (pmd_none(READ_ONCE(*pmd))) {
+		if (pmd_none(pmdp_get(pmd))) {
 			void *p;
 
 			p = vmemmap_alloc_block_buf(PMD_SIZE, node, altmap);

mm/vmscan.c +1 -1

index 0ceed77af0fbd..deeb4310fd54c 100644
@@ -3631,7 +3631,7 @@ static int walk_pud_range(p4d_t *p4d, unsigned long start, unsigned long end,
 	pud = pud_offset(p4d, start & P4D_MASK);
 restart:
 	for (i = pud_index(start), addr = start; addr != end; i++, addr = next) {
-		pud_t val = READ_ONCE(pud[i]);
+		pud_t val = pudp_get(pud + i);
 
 		next = pud_addr_end(addr, end);

Signature · 1 line

2.53.0

Sasha Levin

maintainer

2026-04-01 17:06 UTC #2 lore raw

PATCH 2/2 mm/memory: fix PMD/PUD checks in follow_pfnmap_start()

From: "David Hildenbrand (Arm)" <[email protected]>

[ Upstream commit ffef67b93aa352b34e6aeba3d52c19a63885409a ]

follow_pfnmap_start() suffers from two problems:

(1) We are not re-fetching the pmd/pud after taking the PTL

Properly use pmdp_get() / pudp_get() after taking the lock.

(2) pmd_leaf() / pud_leaf() are not well defined on non-present entries

pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries.

Let's check pmd_present()/pud_present() before assuming "the is a present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page table handling code that traverses user page tables does.

Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, (1) is likely more relevant than (2). It is questionable how often (1) would actually trigger, but let's CC stable to be sure.

This was found by code inspection.

Link https://lkml.kernel.org/r/[email protected] open here

Fixes 6da8e9634bb7 ("mm: new follow_pfnmap API")

Signed-off-by David Hildenbrand (Arm) <[email protected]>

Acked-by Mike Rapoport (Microsoft) <[email protected]>

Reviewed-by Lorenzo Stoakes (Oracle) <[email protected]>

Cc Liam Howlett <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Peter Xu <[email protected]>

Cc Suren Baghdasaryan <[email protected]>

Cc Vlastimil Babka <[email protected]>

Cc <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 mm/memory.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

mm/memory.c +15 -3

index d27cd9a7443ce..49ee03c4392ef 100644
@@ -6457,11 +6457,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pudp = pud_offset(p4dp, address);
 	pud = pudp_get(pudp);
-	if (pud_none(pud))
+	if (!pud_present(pud))
 		goto out;
 	if (pud_leaf(pud)) {
 		lock = pud_lock(mm, pudp);
-		if (!unlikely(pud_leaf(pud))) {
+		pud = pudp_get(pudp);
+
+		if (unlikely(!pud_present(pud))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pud_leaf(pud))) {
 			spin_unlock(lock);
 			goto retry;
 		}

@@ -6473,9 +6478,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pmdp = pmd_offset(pudp, address);
 	pmd = pmdp_get_lockless(pmdp);
+	if (!pmd_present(pmd))
+		goto out;
 	if (pmd_leaf(pmd)) {
 		lock = pmd_lock(mm, pmdp);
-		if (!unlikely(pmd_leaf(pmd))) {
+		pmd = pmdp_get(pmdp);
+
+		if (unlikely(!pmd_present(pmd))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pmd_leaf(pmd))) {
 			spin_unlock(lock);
 			goto retry;
 		}

Signature · 1 line

2.53.0