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mm/ksm: keep soft-dirty when merging a page

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replace_page() maps the KSM page with a PTE built from scratch, so the
soft-dirty bit of the PTE it replaces is lost.  A page written since
"clear_refs 4" then no longer shows as soft-dirty once it is merged, and a
soft-dirty based dumper treats it as unchanged.

Copy the soft-dirty bit from the original PTE to the new PTE.

Link: https://lore.kernel.org/20261021182157.1873726-1-samarth.colleges@gmail.com
Fixes: 0f8975ec4d ("mm: soft-dirty bits for user memory changes tracking")
Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Assisted-by: LLM
Cc: Xu Xin <xu.xin@linux.dev>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Samarth Narang 2026-10-21 11:21:57 -07:00 committed by Andrew Morton
parent c386d799f4
commit 2efc6332f1
1 changed files with 4 additions and 0 deletions
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4
mm/ksm.c
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@ -1463,6 +1463,10 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
dec_mm_counter(mm, MM_ANONPAGES);
}
/* Preserve soft-dirty when replacing the page. */
if (pte_soft_dirty(orig_pte))
newpte = pte_mksoft_dirty(newpte);
flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep)));
/*
* No need to notify as we are replacing a read only page with another