linux-kernel - Re: [PATCH 1/9] mm, swap: use unified helper for swap cache look up

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Message-ID: <CACePvbXWSCm0M-c085ss=ys9XGePQTHocFGfUSm_ZUbemAZwOQ@mail.gmail.com>
Date: Mon, 1 Sep 2025 23:52:08 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <ryncsn@...il.com>
Cc: Barry Song <21cnbao@...il.com>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Hugh Dickins <hughd@...gle.com>, 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 1/9] mm, swap: use unified helper for swap cache look up

On Mon, Sep 1, 2025 at 11:13 PM Kairui Song <ryncsn@...il.com> wrote:
>
> On Tue, Sep 2, 2025 at 9:14 AM Barry Song <21cnbao@...il.com> wrote:
> >
> > On Sat, Aug 23, 2025 at 3:20 AM Kairui Song <ryncsn@...il.com> wrote:
> > >
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > Always use swap_cache_get_folio for swap cache folio look up. The reason
> > > we are not using it in all places is that it also updates the readahead
> > > info, and some callsites want to avoid that.
> > >
> > > So decouple readahead update with swap cache lookup into a standalone
> > > helper, let the caller call the readahead update helper if that's
> > > needed. And convert all swap cache lookups to use swap_cache_get_folio.
> > >
> > > After this commit, there are only three special cases for accessing swap
> > > cache space now: huge memory splitting, migration and shmem replacing,
> > > because they need to lock the Xarray. Following commits will wrap their
> > > accesses to the swap cache too with special helpers.
> > >
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> >
> > Nice! This has cleaned up the confusing mix of using
> > swap_cache_get_folio with VMA, filemap_get_entry,
> > swap_cache_get_folio without VMA, and filemap_get_folio.
> >
> > Reviewed-by: Barry Song <baohua@...nel.org>
>
> Thanks!
>
> >
> > Do we have any potential "dropbehind" cases for anonymous folios?
> > I guess not for now.
> >
>
> Right, dropbehind doesn't apply to anon yet.
>
> > __filemap_get_folio()
> > {
> >         if (folio_test_dropbehind(folio) && !(fgp_flags & FGP_DONTCACHE))
> >                 folio_clear_dropbehind(folio);
> > }
> >
> > Can we mention something about it in the changelog?
>
> I can add some words about it in the commit message. One can easily
> tell that if we want dropbehind for anon, swap_caceh_get_folio will be
> the right place to handle related logics.
>
> Now with swap_cache_get_folio being the only place for swap cache
> lookup, and in the next phase we'll make the swap cache layer the
> unified way to do swap synchronization and never bypass it, maybe
> dropbehind will be easier to do too.

Thanks for the cleaning up and unified swap cache synchronization.

Really appreciate it.

Chris