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

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Message-ID: <CAMgjq7Au2n7PO6yb7wyCJbvFceGj6cpQCp+kVTewPjU=WhmXKQ@mail.gmail.com>
Date: Tue, 2 Sep 2025 14:12:57 +0800
From: Kairui Song <ryncsn@...il.com>
To: Barry Song <21cnbao@...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>, 
	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 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.

>
> Best regards
> Barry
>