The patch titled
Subject: fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Ryan Roberts <ryan.roberts@xxxxxxx>
Subject: fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan
Date: Mon, 29 Apr 2024 12:40:17 +0100
make_uffd_wp_pte() was previously doing:
pte = ptep_get(ptep);
ptep_modify_prot_start(ptep);
pte = pte_mkuffd_wp(pte);
ptep_modify_prot_commit(ptep, pte);
But if another thread accessed or dirtied the pte between the first 2
calls, this could lead to loss of that information. Since
ptep_modify_prot_start() gets and clears atomically, the following is the
correct pattern and prevents any possible race. Any access after the
first call would see an invalid pte and cause a fault:
pte = ptep_modify_prot_start(ptep);
pte = pte_mkuffd_wp(pte);
ptep_modify_prot_commit(ptep, pte);
Link: https://lkml.kernel.org/r/20240429114017.182570-1-ryan.roberts@xxxxxxx
Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/proc/task_mmu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/proc/task_mmu.c~fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan
+++ a/fs/proc/task_mmu.c
@@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_a
pte_t old_pte;
old_pte = ptep_modify_prot_start(vma, addr, pte);
- ptent = pte_mkuffd_wp(ptent);
+ ptent = pte_mkuffd_wp(old_pte);
ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
} else if (is_swap_pte(ptent)) {
ptent = pte_swp_mkuffd_wp(ptent);
_
Patches currently in -mm which might be from ryan.roberts@xxxxxxx are
fs-proc-task_mmu-fix-loss-of-young-dirty-bits-during-pagemap-scan.patch
fs-proc-task_mmu-fix-uffd-wp-confusion-in-pagemap_scan_pmd_entry.patch
selftests-mm-soft-dirty-should-fail-if-a-testcase-fails.patch
mm-fix-race-between-__split_huge_pmd_locked-and-gup-fast.patch
