linux-kernel - Re: [PATCH v3 05/15] mm, swap: always lock and check the swap cache folio before use

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Message-ID: <CACePvbX4juhR5jry0Bi202qo=nfFVZkztHzo8KxKMW_yKBseyA@mail.gmail.com>
Date: Mon, 15 Sep 2025 08:07:32 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>, Barry Song <baohua@...nel.org>, 
	Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Ying Huang <ying.huang@...ux.alibaba.com>, Johannes Weiner <hannes@...xchg.org>, 
	David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	linux-kernel@...r.kernel.org, Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH v3 05/15] mm, swap: always lock and check the swap cache
 folio before use

On Wed, Sep 10, 2025 at 9:09 AM Kairui Song <ryncsn@...il.com> wrote:
> @@ -2004,6 +2002,13 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
>         bool hwpoisoned = false;
>         int ret = 1;
>
> +       /*
> +        * If the folio is removed from swap cache by others, continue to
> +        * unuse other PTEs. try_to_unuse may try again if we missed this one.
> +        */

It took me a while to figure out why we add a
folio_matches_swap_entry() check here but we don't have an existing
check for folio swap entry matching in this function. Can you confirm
that if a race has happened causing the folio swap entry mismatch,
then try_to_usue() MUST try again rather than "may" try again. It
seems to me that it is a MUST rather than "may". I am curious in what
condition the mismatch happens and the try_to_unuse() does not need to
try again.

BTW, this function has three types of return value, 1, 0, and negative
for error. The 0 and 1 are ignored by the caller, the caller only
cares about the negative value. Overall this unuse_pte() and
try_to_unuse() walk feels complicated and maybe a candidate for later
clean up. That is not your patch's fault. I am not requesting a
cleanup in this series.

With that Nitpick,

Acked-by: Chris Li <chrisl@...nel.org>

Chris

> +       if (!folio_matches_swap_entry(folio, entry))
> +               return 0;
> +
>         swapcache = folio;
>         folio = ksm_might_need_to_copy(folio, vma, addr);
>         if (unlikely(!folio))
> --
> 2.51.0
>