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* [PATCH V2 5.10.y 0/2] Fix bad pmd due to race between change_prot_numa() and THP migration
@ 2026-01-07  3:21 Harry Yoo
  2026-01-07  3:21 ` [PATCH V2 5.10.y 1/2] mm/mprotect: use long for page accountings and retval Harry Yoo
  2026-01-07  3:21 ` [PATCH V2 5.10.y 2/2] mm/mprotect: delete pmd_none_or_clear_bad_unless_trans_huge() Harry Yoo
  0 siblings, 2 replies; 6+ messages in thread
From: Harry Yoo @ 2026-01-07  3:21 UTC (permalink / raw)
  To: stable
  Cc: Liam.Howlett, akpm, baohua, baolin.wang, david, dev.jain, hughd,
	jane.chu, jannh, kas, lance.yang, linux-mm, lorenzo.stoakes,
	npache, pfalcato, ryan.roberts, vbabka, ziy, Harry Yoo

V1 -> V2:
  - Because `pmd_val` variable broke ppc builds due to its name,
    renamed it to `_pmd`. see [1].
    [1] https://lore.kernel.org/stable/aS7lPZPYuChOTdXU@hyeyoo

  - Added David Hildenbrand's Acked-by [2], thanks a lot!
    [2] https://lore.kernel.org/linux-mm/ac8d7137-3819-4a75-9dd3-fb3d2259ebe4@kernel.org/

# TL;DR

previous discussion: https://lore.kernel.org/linux-mm/20250921232709.1608699-1-harry.yoo@oracle.com/

A "bad pmd" error occurs due to race condition between
change_prot_numa() and THP migration. The mainline kernel does not have
this bug as commit 670ddd8cdc fixes the race condition. 6.1.y, 5.15.y,
5.10.y, 5.4.y are affected by this bug. 

Fixing this in -stable kernels is tricky because pte_map_offset_lock()
has different semantics in pre-6.5 and post-6.5 kernels. I am trying to
backport the same mechanism we have in the mainline kernel.
Since the code looks bit different due to different semantics of
pte_map_offset_lock(), it'd be best to get this reviewed by MM folks.

# Testing

I verified that the bug described below is not reproduced anymore
(on a downstream kernel) after applying this patch series. It used to
trigger in few days of intensive numa balancing testing, but it survived
2 weeks with this applied.

# Bug Description

It was reported that a bad pmd is seen when automatic NUMA
balancing is marking page table entries as prot_numa:
    
  [2437548.196018] mm/pgtable-generic.c:50: bad pmd 00000000af22fc02(dffffffe71fbfe02)
  [2437548.235022] Call Trace:
  [2437548.238234]  <TASK>
  [2437548.241060]  dump_stack_lvl+0x46/0x61
  [2437548.245689]  panic+0x106/0x2e5
  [2437548.249497]  pmd_clear_bad+0x3c/0x3c
  [2437548.253967]  change_pmd_range.isra.0+0x34d/0x3a7
  [2437548.259537]  change_p4d_range+0x156/0x20e
  [2437548.264392]  change_protection_range+0x116/0x1a9
  [2437548.269976]  change_prot_numa+0x15/0x37
  [2437548.274774]  task_numa_work+0x1b8/0x302
  [2437548.279512]  task_work_run+0x62/0x95
  [2437548.283882]  exit_to_user_mode_loop+0x1a4/0x1a9
  [2437548.289277]  exit_to_user_mode_prepare+0xf4/0xfc
  [2437548.294751]  ? sysvec_apic_timer_interrupt+0x34/0x81
  [2437548.300677]  irqentry_exit_to_user_mode+0x5/0x25
  [2437548.306153]  asm_sysvec_apic_timer_interrupt+0x16/0x1b

This is due to a race condition between change_prot_numa() and
THP migration because the kernel doesn't check is_swap_pmd() and
pmd_trans_huge() atomically:

change_prot_numa()                      THP migration
======================================================================
- change_pmd_range()
-> is_swap_pmd() returns false,
meaning it's not a PMD migration
entry.
				  - do_huge_pmd_numa_page()
				  -> migrate_misplaced_page() sets
				     migration entries for the THP.
- change_pmd_range()
-> pmd_none_or_clear_bad_unless_trans_huge()
-> pmd_none() and pmd_trans_huge() returns false
- pmd_none_or_clear_bad_unless_trans_huge()
-> pmd_bad() returns true for the migration entry!

The upstream commit 670ddd8cdcbd ("mm/mprotect: delete
pmd_none_or_clear_bad_unless_trans_huge()") closes this race condition
by checking is_swap_pmd() and pmd_trans_huge() atomically.

# Backporting note

commit a79390f5d6a7 ("mm/mprotect: use long for page accountings and retval")
is backported to return an error code (negative value) in
change_pte_range().

Unlike the mainline, pte_offset_map_lock() does not check if the pmd
entry is a migration entry or a hugepage; acquires PTL unconditionally
instead of returning failure. Therefore, it is necessary to keep the
!is_swap_pmd() && !pmd_trans_huge() && !pmd_devmap() checks in
change_pmd_range() before acquiring the PTL.

After acquiring the lock, open-code the semantics of
pte_offset_map_lock() in the mainline kernel; change_pte_range() fails
if the pmd value has changed. This requires adding pmd_old parameter
(pmd_t value that is read before calling the function) to
change_pte_range().

Hugh Dickins (1):
  mm/mprotect: delete pmd_none_or_clear_bad_unless_trans_huge()

Peter Xu (1):
  mm/mprotect: use long for page accountings and retval

 include/linux/hugetlb.h |   4 +-
 include/linux/mm.h      |   2 +-
 mm/hugetlb.c            |   4 +-
 mm/mempolicy.c          |   2 +-
 mm/mprotect.c           | 107 ++++++++++++++++++++++------------------
 5 files changed, 64 insertions(+), 55 deletions(-)

-- 
2.43.0



^ permalink raw reply	[flat|nested] 6+ messages in thread
* [PATCH V1 5.10.y 1/2] mm/mprotect: use long for page accountings and retval
@ 2025-11-25  4:54 Harry Yoo
  2025-11-27 14:02 ` Patch "mm/mprotect: use long for page accountings and retval" has been added to the 5.10-stable tree gregkh
  0 siblings, 1 reply; 6+ messages in thread
From: Harry Yoo @ 2025-11-25  4:54 UTC (permalink / raw)
  To: stable
  Cc: Liam.Howlett, akpm, baohua, baolin.wang, david, dev.jain, hughd,
	jane.chu, jannh, kas, lance.yang, linux-mm, lorenzo.stoakes,
	npache, pfalcato, ryan.roberts, vbabka, ziy, Peter Xu,
	Mike Kravetz, James Houghton, Andrea Arcangeli, Axel Rasmussen,
	David Hildenbrand, Muchun Song, Nadav Amit, Harry Yoo

From: Peter Xu <peterx@redhat.com>

commit a79390f5d6a78647fd70856bd42b22d994de0ba2 upstream.

Switch to use type "long" for page accountings and retval across the whole
procedure of change_protection().

The change should have shrinked the possible maximum page number to be
half comparing to previous (ULONG_MAX / 2), but it shouldn't overflow on
any system either because the maximum possible pages touched by change
protection should be ULONG_MAX / PAGE_SIZE.

Two reasons to switch from "unsigned long" to "long":

  1. It suites better on count_vm_numa_events(), whose 2nd parameter takes
     a long type.

  2. It paves way for returning negative (error) values in the future.

Currently the only caller that consumes this retval is change_prot_numa(),
where the unsigned long was converted to an int.  Since at it, touching up
the numa code to also take a long, so it'll avoid any possible overflow
too during the int-size convertion.

Link: https://lkml.kernel.org/r/20230104225207.1066932-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: James Houghton <jthoughton@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
[ Adjust context ]
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---
 include/linux/hugetlb.h |  4 ++--
 include/linux/mm.h      |  2 +-
 mm/hugetlb.c            |  4 ++--
 mm/mempolicy.c          |  2 +-
 mm/mprotect.c           | 34 +++++++++++++++++-----------------
 5 files changed, 23 insertions(+), 23 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 1c03935aa3d13..f4d20096959b2 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -184,7 +184,7 @@ struct page *follow_huge_pgd(struct mm_struct *mm, unsigned long address,
 
 int pmd_huge(pmd_t pmd);
 int pud_huge(pud_t pud);
-unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
+long hugetlb_change_protection(struct vm_area_struct *vma,
 		unsigned long address, unsigned long end, pgprot_t newprot);
 
 bool is_hugetlb_entry_migration(pte_t pte);
@@ -342,7 +342,7 @@ static inline void move_hugetlb_state(struct page *oldpage,
 {
 }
 
-static inline unsigned long hugetlb_change_protection(
+static inline long hugetlb_change_protection(
 			struct vm_area_struct *vma, unsigned long address,
 			unsigned long end, pgprot_t newprot)
 {
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e168d87d6f2ee..0d5b9efc73389 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1876,7 +1876,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
 #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
 					    MM_CP_UFFD_WP_RESOLVE)
 
-extern unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
+extern long change_protection(struct vm_area_struct *vma, unsigned long start,
 			      unsigned long end, pgprot_t newprot,
 			      unsigned long cp_flags);
 extern int mprotect_fixup(struct vm_area_struct *vma,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8efe35ea0baa7..ef181edabefe5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5051,7 +5051,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 #define flush_hugetlb_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
 #endif
 
-unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
+long hugetlb_change_protection(struct vm_area_struct *vma,
 		unsigned long address, unsigned long end, pgprot_t newprot)
 {
 	struct mm_struct *mm = vma->vm_mm;
@@ -5059,7 +5059,7 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
 	pte_t *ptep;
 	pte_t pte;
 	struct hstate *h = hstate_vma(vma);
-	unsigned long pages = 0;
+	long pages = 0;
 	bool shared_pmd = false;
 	struct mmu_notifier_range range;
 
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 6c98585f20dfe..59ccda77d2fca 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -653,7 +653,7 @@ static int queue_pages_hugetlb(pte_t *pte, unsigned long hmask,
 unsigned long change_prot_numa(struct vm_area_struct *vma,
 			unsigned long addr, unsigned long end)
 {
-	int nr_updated;
+	long nr_updated;
 
 	nr_updated = change_protection(vma, addr, end, PAGE_NONE, MM_CP_PROT_NUMA);
 	if (nr_updated)
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 7ea0aee0c08d9..28e1a8fd9319f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -35,13 +35,13 @@
 
 #include "internal.h"
 
-static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
+static long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long addr, unsigned long end, pgprot_t newprot,
 		unsigned long cp_flags)
 {
 	pte_t *pte, oldpte;
 	spinlock_t *ptl;
-	unsigned long pages = 0;
+	long pages = 0;
 	int target_node = NUMA_NO_NODE;
 	bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT;
 	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
@@ -209,13 +209,13 @@ static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd)
 	return 0;
 }
 
-static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
+static inline long change_pmd_range(struct vm_area_struct *vma,
 		pud_t *pud, unsigned long addr, unsigned long end,
 		pgprot_t newprot, unsigned long cp_flags)
 {
 	pmd_t *pmd;
 	unsigned long next;
-	unsigned long pages = 0;
+	long pages = 0;
 	unsigned long nr_huge_updates = 0;
 	struct mmu_notifier_range range;
 
@@ -223,7 +223,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
 
 	pmd = pmd_offset(pud, addr);
 	do {
-		unsigned long this_pages;
+		long this_pages;
 
 		next = pmd_addr_end(addr, end);
 
@@ -281,13 +281,13 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
 	return pages;
 }
 
-static inline unsigned long change_pud_range(struct vm_area_struct *vma,
-		p4d_t *p4d, unsigned long addr, unsigned long end,
-		pgprot_t newprot, unsigned long cp_flags)
+static inline long change_pud_range(struct vm_area_struct *vma, p4d_t *p4d,
+		unsigned long addr, unsigned long end, pgprot_t newprot,
+		unsigned long cp_flags)
 {
 	pud_t *pud;
 	unsigned long next;
-	unsigned long pages = 0;
+	long pages = 0;
 
 	pud = pud_offset(p4d, addr);
 	do {
@@ -301,13 +301,13 @@ static inline unsigned long change_pud_range(struct vm_area_struct *vma,
 	return pages;
 }
 
-static inline unsigned long change_p4d_range(struct vm_area_struct *vma,
-		pgd_t *pgd, unsigned long addr, unsigned long end,
-		pgprot_t newprot, unsigned long cp_flags)
+static inline long change_p4d_range(struct vm_area_struct *vma, pgd_t *pgd,
+		unsigned long addr, unsigned long end, pgprot_t newprot,
+		unsigned long cp_flags)
 {
 	p4d_t *p4d;
 	unsigned long next;
-	unsigned long pages = 0;
+	long pages = 0;
 
 	p4d = p4d_offset(pgd, addr);
 	do {
@@ -321,7 +321,7 @@ static inline unsigned long change_p4d_range(struct vm_area_struct *vma,
 	return pages;
 }
 
-static unsigned long change_protection_range(struct vm_area_struct *vma,
+static long change_protection_range(struct vm_area_struct *vma,
 		unsigned long addr, unsigned long end, pgprot_t newprot,
 		unsigned long cp_flags)
 {
@@ -329,7 +329,7 @@ static unsigned long change_protection_range(struct vm_area_struct *vma,
 	pgd_t *pgd;
 	unsigned long next;
 	unsigned long start = addr;
-	unsigned long pages = 0;
+	long pages = 0;
 
 	BUG_ON(addr >= end);
 	pgd = pgd_offset(mm, addr);
@@ -351,11 +351,11 @@ static unsigned long change_protection_range(struct vm_area_struct *vma,
 	return pages;
 }
 
-unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
+long change_protection(struct vm_area_struct *vma, unsigned long start,
 		       unsigned long end, pgprot_t newprot,
 		       unsigned long cp_flags)
 {
-	unsigned long pages;
+	long pages;
 
 	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
 
-- 
2.43.0



^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2026-01-08 16:28 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2026-01-07  3:21 [PATCH V2 5.10.y 0/2] Fix bad pmd due to race between change_prot_numa() and THP migration Harry Yoo
2026-01-07  3:21 ` [PATCH V2 5.10.y 1/2] mm/mprotect: use long for page accountings and retval Harry Yoo
2026-01-08 16:27   ` Patch "mm/mprotect: use long for page accountings and retval" has been added to the 5.10-stable tree gregkh
2026-01-07  3:21 ` [PATCH V2 5.10.y 2/2] mm/mprotect: delete pmd_none_or_clear_bad_unless_trans_huge() Harry Yoo
2026-01-08 16:27   ` Patch "mm/mprotect: delete pmd_none_or_clear_bad_unless_trans_huge()" has been added to the 5.10-stable tree gregkh
  -- strict thread matches above, loose matches on Subject: below --
2025-11-25  4:54 [PATCH V1 5.10.y 1/2] mm/mprotect: use long for page accountings and retval Harry Yoo
2025-11-27 14:02 ` Patch "mm/mprotect: use long for page accountings and retval" has been added to the 5.10-stable tree gregkh

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