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

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Message-ID: <dd6249d3-3a2c-4eda-9b3f-bcf72a4d36e4@lucifer.local>
Date: Tue, 28 Oct 2025 16:41:42 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Barry Song <baohua@...nel.org>,
        "open list:MEMORY MAPPING" <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/mremap: Honour writable bit in mremap pte batching

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@...r.kernel.org #6.17
> Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching")
> Reported-by: David Hildenbrand <david@...hat.com>
> Debugged-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Dev Jain <dev.jain@....com>

LGTM, so:

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