[PATCH v3] mm/ksm: keep soft-dirty when merging a page

[Samarth Narang]

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.

Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Samarth Narang <samarth.colleges@xxxxxxxxx>
---

Notes:
    v2: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@xxxxxxxxx/
    
    Changes since v2:
    - Keep only the soft-dirty fix, as David suggested; Lance will be handling
      the uffd-wp/RWP part.
    - Fixes tag reduced to the soft-dirty commit.
    - Dropped the Tested-by tag, since the patch changed.

 mm/ksm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index dbfb8f98b189..910a1307ac9f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -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

base-commit: fcec4f4bcc1101dbd7e3aff267856b3bbfe128c6
-- 
2.53.0