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...

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@arm.com 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@arm.com Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs") Signed-off-by: Ryan Roberts ryan.roberts@arm.com Acked-by: David Hildenbrand david@redhat.com Cc: Muhammad Usama Anjum usama.anjum@collabora.com Cc: Peter Xu peterx@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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@arm.com 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