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

Lorenzo Stoakes
2025-10-28T16:42:02Z

You'll probably get a tag just from using the British English spelling of
'honour' from me :P (joking! ;)

On Tue, Oct 28, 2025 at 12:09:52PM +0530, Dev Jain wrote:
> 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.

Yikes.

>
> 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>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
> mm-selftests pass. Based on mm-new. Need David H. to confirm whether
> the repro passes.

Given he A-b'd I assume it did :)

>
> 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
>