* + mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch added to mm-hotfixes-unstable branch
@ 2026-03-17 17:50 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-03-17 17:50 UTC (permalink / raw)
To: mm-commits, vbabka, surenb, stable, rppt, mhocko, ljs,
liam.howlett, david, mboone, akpm
The patch titled
Subject: mm/pagewalk: fix race between concurrent split and refault
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: Max Boone <mboone@akamai.com>
Subject: mm/pagewalk: fix race between concurrent split and refault
Date: Tue, 17 Mar 2026 15:03:04 +0100
The splitting of a PUD entry in walk_pud_range() can race with a
concurrent thread refaulting the PUD leaf entry causing it to try walking
a PMD range that has disappeared.
An example and reproduction of this is to try reading numa_maps of a
process while VFIO-PCI is setting up DMA (specifically the
vfio_pin_pages_remote call) on a large BAR for that process.
This will trigger a kernel BUG:
vfio-pci 0000:03:00.0: enabling device (0000 -> 0002)
BUG: unable to handle page fault for address: ffffa23980000000
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
...
RIP: 0010:walk_pgd_range+0x3b5/0x7a0
Code: 8d 43 ff 48 89 44 24 28 4d 89 ce 4d 8d a7 00 00 20 00 48 8b 4c 24
28 49 81 e4 00 00 e0 ff 49 8d 44 24 ff 48 39 c8 4c 0f 43 e3 <49> f7 06
9f ff ff ff 75 3b 48 8b 44 24 20 48 8b 40 28 48 85 c0 74
RSP: 0018:ffffac23e1ecf808 EFLAGS: 00010287
RAX: 00007f44c01fffff RBX: 00007f4500000000 RCX: 00007f44ffffffff
RDX: 0000000000000000 RSI: 000ffffffffff000 RDI: ffffffff93378fe0
RBP: ffffac23e1ecf918 R08: 0000000000000004 R09: ffffa23980000000
R10: 0000000000000020 R11: 0000000000000004 R12: 00007f44c0200000
R13: 00007f44c0000000 R14: ffffa23980000000 R15: 00007f44c0000000
FS: 00007fe884739580(0000) GS:ffff9b7d7a9c0000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffa23980000000 CR3: 000000c0650e2005 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
<TASK>
__walk_page_range+0x195/0x1b0
walk_page_vma+0x62/0xc0
show_numa_map+0x12b/0x3b0
seq_read_iter+0x297/0x440
seq_read+0x11d/0x140
vfs_read+0xc2/0x340
ksys_read+0x5f/0xe0
do_syscall_64+0x68/0x130
? get_page_from_freelist+0x5c2/0x17e0
? mas_store_prealloc+0x17e/0x360
? vma_set_page_prot+0x4c/0xa0
? __alloc_pages_noprof+0x14e/0x2d0
? __mod_memcg_lruvec_state+0x8d/0x140
? __lruvec_stat_mod_folio+0x76/0xb0
? __folio_mod_stat+0x26/0x80
? do_anonymous_page+0x705/0x900
? __handle_mm_fault+0xa8d/0x1000
? __count_memcg_events+0x53/0xf0
? handle_mm_fault+0xa5/0x360
? do_user_addr_fault+0x342/0x640
? arch_exit_to_user_mode_prepare.constprop.0+0x16/0xa0
? irqentry_exit_to_user_mode+0x24/0x100
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fe88464f47e
Code: c0 e9 b6 fe ff ff 50 48 8d 3d be 07 0b 00 e8 69 01 02 00 66 0f 1f
84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00
f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28
RSP: 002b:00007ffe6cd9a9b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fe88464f47e
RDX: 0000000000020000 RSI: 00007fe884543000 RDI: 0000000000000003
RBP: 00007fe884543000 R08: 00007fe884542010 R09: 0000000000000000
R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
</TASK>
Fix this by validating the PUD entry in walk_pmd_range() using a stable
snapshot (pudp_get()). If the PUD is not present or is a leaf, retry the
walk via ACTION_AGAIN instead of descending further. This mirrors the
retry logic in walk_pmd_range().
Link: https://lkml.kernel.org/r/20260317-pagewalk-check-pmd-refault-v1-1-f699a010f2b3@akamai.com
Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Max Boone <mboone@akamai.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/pagewalk.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
--- a/mm/pagewalk.c~mm-pagewalk-fix-race-between-concurrent-split-and-refault
+++ a/mm/pagewalk.c
@@ -97,6 +97,7 @@ static int walk_pte_range(pmd_t *pmd, un
static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
struct mm_walk *walk)
{
+ pud_t pudval = pudp_get(pud);
pmd_t *pmd;
unsigned long next;
const struct mm_walk_ops *ops = walk->ops;
@@ -105,6 +106,18 @@ static int walk_pmd_range(pud_t *pud, un
int err = 0;
int depth = real_depth(3);
+ /*
+ * For PTE handling, pte_offset_map_lock() takes care of checking
+ * whether there actually is a page table. But it also has to be
+ * very careful about concurrent page table reclaim. If we spot a PMD
+ * table, it cannot go away, so we can just walk it. However, if we find
+ * something else, we have to retry.
+ */
+ if (!pud_present(pudval) || pud_leaf(pudval)) {
+ walk->action = ACTION_AGAIN;
+ return 0;
+ }
+
pmd = pmd_offset(pud, addr);
do {
again:
@@ -218,12 +231,13 @@ static int walk_pud_range(p4d_t *p4d, un
else if (pud_leaf(*pud) || !pud_present(*pud))
continue; /* Nothing to do. */
- if (pud_none(*pud))
- goto again;
-
err = walk_pmd_range(pud, addr, next, walk);
if (err)
break;
+
+ if (walk->action == ACTION_AGAIN)
+ goto again;
+
} while (pud++, addr = next, addr != end);
return err;
_
Patches currently in -mm which might be from mboone@akamai.com are
mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
* + mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch added to mm-hotfixes-unstable branch
@ 2026-03-26 0:50 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-03-26 0:50 UTC (permalink / raw)
To: mm-commits, vbabka, surenb, stable, rppt, mhocko, ljs,
liam.howlett, david, mboone, akpm
The patch titled
Subject: mm/pagewalk: fix race between concurrent split and refault
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: Max Boone <mboone@akamai.com>
Subject: mm/pagewalk: fix race between concurrent split and refault
Date: Wed, 25 Mar 2026 10:59:16 +0100
The splitting of a PUD entry in walk_pud_range() can race with a
concurrent thread refaulting the PUD leaf entry causing it to try walking
a PMD range that has disappeared.
An example and reproduction of this is to try reading numa_maps of a
process while VFIO-PCI is setting up DMA (specifically the
vfio_pin_pages_remote call) on a large BAR for that process.
This will trigger a kernel BUG:
vfio-pci 0000:03:00.0: enabling device (0000 -> 0002)
BUG: unable to handle page fault for address: ffffa23980000000
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
...
RIP: 0010:walk_pgd_range+0x3b5/0x7a0
Code: 8d 43 ff 48 89 44 24 28 4d 89 ce 4d 8d a7 00 00 20 00 48 8b 4c 24
28 49 81 e4 00 00 e0 ff 49 8d 44 24 ff 48 39 c8 4c 0f 43 e3 <49> f7 06
9f ff ff ff 75 3b 48 8b 44 24 20 48 8b 40 28 48 85 c0 74
RSP: 0018:ffffac23e1ecf808 EFLAGS: 00010287
RAX: 00007f44c01fffff RBX: 00007f4500000000 RCX: 00007f44ffffffff
RDX: 0000000000000000 RSI: 000ffffffffff000 RDI: ffffffff93378fe0
RBP: ffffac23e1ecf918 R08: 0000000000000004 R09: ffffa23980000000
R10: 0000000000000020 R11: 0000000000000004 R12: 00007f44c0200000
R13: 00007f44c0000000 R14: ffffa23980000000 R15: 00007f44c0000000
FS: 00007fe884739580(0000) GS:ffff9b7d7a9c0000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffa23980000000 CR3: 000000c0650e2005 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
<TASK>
__walk_page_range+0x195/0x1b0
walk_page_vma+0x62/0xc0
show_numa_map+0x12b/0x3b0
seq_read_iter+0x297/0x440
seq_read+0x11d/0x140
vfs_read+0xc2/0x340
ksys_read+0x5f/0xe0
do_syscall_64+0x68/0x130
? get_page_from_freelist+0x5c2/0x17e0
? mas_store_prealloc+0x17e/0x360
? vma_set_page_prot+0x4c/0xa0
? __alloc_pages_noprof+0x14e/0x2d0
? __mod_memcg_lruvec_state+0x8d/0x140
? __lruvec_stat_mod_folio+0x76/0xb0
? __folio_mod_stat+0x26/0x80
? do_anonymous_page+0x705/0x900
? __handle_mm_fault+0xa8d/0x1000
? __count_memcg_events+0x53/0xf0
? handle_mm_fault+0xa5/0x360
? do_user_addr_fault+0x342/0x640
? arch_exit_to_user_mode_prepare.constprop.0+0x16/0xa0
? irqentry_exit_to_user_mode+0x24/0x100
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fe88464f47e
Code: c0 e9 b6 fe ff ff 50 48 8d 3d be 07 0b 00 e8 69 01 02 00 66 0f 1f
84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00
f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28
RSP: 002b:00007ffe6cd9a9b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fe88464f47e
RDX: 0000000000020000 RSI: 00007fe884543000 RDI: 0000000000000003
RBP: 00007fe884543000 R08: 00007fe884542010 R09: 0000000000000000
R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
</TASK>
Fix this by validating the PUD entry in walk_pmd_range() using a stable
snapshot (pudp_get()). If the PUD is not present or is a leaf, retry the
walk via ACTION_AGAIN instead of descending further. This mirrors the
retry logic in walk_pte_range(), which lets walk_pmd_range() retry if the
PTE is not being got by pte_offset_map_lock().
Link: https://lkml.kernel.org/r/20260325-pagewalk-check-pmd-refault-v2-1-707bff33bc60@akamai.com
Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support")
Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Max Boone <mboone@akamai.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/pagewalk.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)
--- a/mm/pagewalk.c~mm-pagewalk-fix-race-between-concurrent-split-and-refault
+++ a/mm/pagewalk.c
@@ -97,6 +97,7 @@ static int walk_pte_range(pmd_t *pmd, un
static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
struct mm_walk *walk)
{
+ pud_t pudval = pudp_get(pud);
pmd_t *pmd;
unsigned long next;
const struct mm_walk_ops *ops = walk->ops;
@@ -105,6 +106,24 @@ static int walk_pmd_range(pud_t *pud, un
int err = 0;
int depth = real_depth(3);
+ /*
+ * For PTE handling, pte_offset_map_lock() takes care of checking
+ * whether there actually is a page table. But it also has to be
+ * very careful about concurrent page table reclaim.
+ *
+ * Similarly, we have to be careful here - a PUD entry that points
+ * to a PMD table cannot go away, so we can just walk it. But if
+ * it's something else, we need to ensure we didn't race something,
+ * so need to retry.
+ *
+ * A pertinent example of this is a PUD refault after PUD split -
+ * we will need to split again or risk accessing invalid memory.
+ */
+ if (!pud_present(pudval) || pud_leaf(pudval)) {
+ walk->action = ACTION_AGAIN;
+ return 0;
+ }
+
pmd = pmd_offset(pud, addr);
do {
again:
@@ -218,12 +237,12 @@ static int walk_pud_range(p4d_t *p4d, un
else if (pud_leaf(*pud) || !pud_present(*pud))
continue; /* Nothing to do. */
- if (pud_none(*pud))
- goto again;
-
err = walk_pmd_range(pud, addr, next, walk);
if (err)
break;
+
+ if (walk->action == ACTION_AGAIN)
+ goto again;
} while (pud++, addr = next, addr != end);
return err;
_
Patches currently in -mm which might be from mboone@akamai.com are
mm-pagewalk-fix-race-between-concurrent-split-and-refault.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
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