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

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Message-ID: <CAKEwX=Ndtbsy7UoGhwcd+by6bz2gt82o6EPOkiXbqNv=uPb8wg@mail.gmail.com>
Date: Fri, 5 Sep 2025 15:40:16 -0700
From: Nhat Pham <nphamcs@...il.com>
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>, Chris Li <chrisl@...nel.org>, 
	Barry Song <baohua@...nel.org>, Baoquan He <bhe@...hat.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

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

I mean if there is a race, folio->swap could literally be anything
right? Can the following happen: between the filemap_get_folio()
lookup and the locking, the folio can have its swap slot freed up,
then obtain a new swap slot, potentially from an entirely different
swapfile (i.e different swp_type(folio->swap)).

It is very unlikely, and in many setups you only have one swapfile. Still...

> 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>

Yeah that's pretty nuanced lol. It is unlikely to cause any issue
indeed - we're just occasionally swap-cache-reclaim some rando folio
haha.

Anyway, FWIW:

Acked-by: Nhat Pham <nphamcs@...il.com>