[PATCH] mm/mremap: Honour writable bit in mremap pte batching

Dev Jain
2025-10-28T06:40:27Z

Currently mremap folio pte batch ignores the writable bit during figuring
out a set of similar ptes mapping the same folio. Suppose that the first
pte of the batch is writable while the others are not - set_ptes will
end up setting the writable bit on the other ptes, which is a violation
of mremap semantics. Therefore, use FPB_RESPECT_WRITE to check the writable
bit while determining the pte batch.

Cc: stable@vger.kernel.org #6.17
Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching")
Reported-by: David Hildenbrand <david@redhat.com>
Debugged-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Dev Jain <dev.jain@arm.com>
---
mm-selftests pass. Based on mm-new. Need David H. to confirm whether
the repro passes.

mm/mremap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index a7f531c17b79..8ad06cf50783 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -187,7 +187,7 @@ static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr
if (!folio || !folio_test_large(folio))
return 1;

- return folio_pte_batch(folio, ptep, pte, max_nr);
+ return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr, FPB_RESPECT_WRITE);
}

static int move_ptes(struct pagetable_move_control *pmc,
--
2.30.2