[PATCH 6.1.y] mm/ksm: keep soft-dirty when merging a page

[Sasha Levin <sashal@kernel.org>]

From: Samarth Narang <samarth.colleges@gmail.com>

[ Upstream commit 2efc6332f18bfd848f74f3d5ba45b31450601faa ]

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: 0f8975ec4db2 ("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>
[ sashal: Adjusted context to retain pte_pfn(*ptep) instead of pte_pfn(ptep_get(ptep)). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/ksm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 99eb19860fd59..c976e41f853b0 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1178,6 +1178,10 @@ 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));
 	/*
 	 * No need to notify as we are replacing a read only page with another
-- 
2.53.0