linux-kernel - Re: [PATCH v2 03/15] mm, swap: fix swap cahe index error when retrying reclaim

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Message-ID: <CAF8kJuNhLDovRO+8Y25ArdMBVFjXnYBVjmASzMBtdsdZkQ0NLQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 18:51:55 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <kasong@...cent.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
Subject: Re: [PATCH v2 03/15] mm, swap: fix swap cahe index error when
 retrying reclaim

Hi Kairui,

The patch looks obviously correct to me with some very minor nitpicks following.

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

On Fri, Sep 5, 2025 at 12:14 PM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> The allocator will reclaim cached slots while scanning. Currently, it
> will try again if the reclaim found a folio that is already removed from
> the swap cache due to a race. But the following lookup will be using the
> wrong index. It won't cause any OOB issue since the swap cache index is
> truncated upon lookup, but it may lead to reclaiming of an irrelevant
> folio.
>
> This should not cause a measurable issue, but we should fix it.
>
> Fixes: fae8595505313 ("mm, swap: avoid reclaiming irrelevant swap cache")
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swapfile.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4b8ab2cb49ca..4c63fc62f4cb 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -240,13 +240,13 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
>          * Offset could point to the middle of a large folio, or folio
>          * may no longer point to the expected offset before it's locked.
>          */
> -       entry = folio->swap;
Nitpick:
This and the following reuse the folio->swap dereference and
swp_offset() many times.
You can use some local variables to cache the value into a register
and less function calls. I haven't looked into if the compiler will do
the same expression elimination on this, a good compiler should. The
following looks less busy and doesn't need the compiler to optimize it
for you.

           fe = folio->swap;
           eoffset = swp_offset(fe);
           if (offset < eoffset ) || offset >= eoffset + nr_pages) {
...
           }
           offset = eoffset;

This might generate better code due to less function code. If the
compiler does the perfect jobs the original code can generate the same
optimized code as well.

> -       if (offset < swp_offset(entry) || offset >= swp_offset(entry) + nr_pages) {
> +       if (offset < swp_offset(folio->swap) ||
> +           offset >= swp_offset(folio->swap) + nr_pages) {
>                 folio_unlock(folio);
>                 folio_put(folio);
>                 goto again;
>         }
> -       offset = swp_offset(entry);
> +       offset = swp_offset(folio->swap);

So the first entry is only assigned once in the function and never changed?

You can use const to declare it.

Chris

>
>         need_reclaim = ((flags & TTRS_ANYWAY) ||
>                         ((flags & TTRS_UNMAPPED) && !folio_mapped(folio)) ||
> --
> 2.51.0
>
>