linux-kernel - Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio before use

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Message-ID: <CAMgjq7BPUE_+AhF_Nm98k6hbVPpLkQcwPGGMqA3yZuKmMR6xZw@mail.gmail.com>
Date: Tue, 2 Sep 2025 21:27:27 +0800
From: Kairui Song <ryncsn@...il.com>
To: Chris Li <chrisl@...nel.org>
Cc: David Hildenbrand <david@...hat.com>, 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>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] mm, swap: always lock and check the swap cache folio
 before use

On Tue, Sep 2, 2025 at 9:03 PM Chris Li <chrisl@...nel.org> wrote:
>
> On Tue, Sep 2, 2025 at 3:18 AM David Hildenbrand <david@...hat.com> wrote:
> >
> > On 22.08.25 21:20, Kairui Song wrote:
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > Swap cache lookup is lockless, it only increases the reference count
> > > of the returned folio. That's not enough to ensure a folio is stable in
> > > the swap cache, so the folio could be removed from the swap cache at any
> > > time. The caller always has to lock and check the folio before use.
> > >
> > > Document this as a comment, and introduce a helper for swap cache folio
> > > verification with proper sanity checks.
> > >
> > > Also, sanitize all current users to use this convention, and use the new
> > > helper when possible for easier debugging. Some existing callers won't
> > > cause any major problem right now, only trivial issues like incorrect
> > > readahead statistic (swapin) or wasted loop (swapoff). It's better to
> > > always follow this convention to make things robust.
> > >
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > ---
> >
> > [...]
> >
> > > +/**
> > > + * folio_contains_swap - Does this folio contain this swap entry?
> > > + * @folio: The folio.
> > > + * @entry: The swap entry to check against.
> > > + *
> > > + * Swap version of folio_contains()
> > > + *
> > > + * Context: The caller should have the folio locked to ensure
> > > + * nothing will move it out of the swap cache.
> > > + * Return: true or false.
> > > + */
> >
> > I appreciate the kerneldoc.
> >
> > Intuitively, this should be called "..._swap_entry".
> >
> > But I wonder if "contains" is really the right term to use here. It's
> > more like that a swap entry "belongs to" (was assigned to) a folio, right?
>
> Right, in the other design doc I use the word "binding" for the
> relationship between folio and swap entry. As if it is a binding
> contract, your folio data goes and only goes here. There is no owning
> relationship. Other folios might want to compete and win over the
> binding contract as well (the race in swap in).
>
> > Sure, we store the information in the folio, but the "contains" is a bit
> > weird.
> >
> > folio_matches_swp_entry() maybe?
>
> Yes, I like the name folio_match_swap_entry() you suggested in the
> other email as well.

I like this name too. The `folio_contains_swap` name comes from
`folio_contains` as it's just like a swap version of it. But I also
found the name a bit strange as they are different things, but had no
better idea. Thanks for the suggestion.

>
> Chris
>