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

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Message-ID: <CAMgjq7BTVCc0yY8PunYZnCjnuybAEZMWvJ8+89Wb_EjXnuC78g@mail.gmail.com>
Date: Sat, 6 Sep 2025 14:30:09 +0800
From: Kairui Song <ryncsn@...il.com>
To: Nhat Pham <nphamcs@...il.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 Sat, Sep 6, 2025 at 7:12 AM Nhat Pham <nphamcs@...il.com> wrote:
>
> 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...

Yeah, but fortunately nothing under
the `again:` will touch the address_space, here so a random value only
causes a random lookup offset in a valid addresss_space, which is
completely fine.



>
> > 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>

Thanks.