linux-kernel - Re: [PATCH 5/9] mm/shmem, swap: remove redundant error handling for replacing folio

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Message-ID: <CACePvbV=HkuP8w=LvifvAv1O8VD0V0XPOfckVtLiEfHS3NnhZA@mail.gmail.com>
Date: Fri, 29 Aug 2025 19:41:20 -0700
From: Chris Li <chrisl@...nel.org>
To: Kairui Song <ryncsn@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.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>, 
	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 5/9] mm/shmem, swap: remove redundant error handling for
 replacing folio

On Mon, Aug 25, 2025 at 2:46 AM Kairui Song <ryncsn@...il.com> wrote:
>
> On Mon, Aug 25, 2025 at 11:09 AM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
> >
> >
> >
> > On 2025/8/23 03:20, Kairui Song wrote:
> > > From: Kairui Song <kasong@...cent.com>
> > >
> > > Shmem may replace a folio in the swap cache if the cached one doesn't
> > > fit the swapin's GFP zone. When doing so, shmem has already double
> > > checked that the swap cache folio is locked, still has the swap cache
> > > flag set, and contains the wanted swap entry. So it is impossible to
> > > fail due to an Xarray mismatch. There is even a comment for that.
> > >
> > > Delete the defensive error handling path, and add a WARN_ON instead:
> > > if that happened, something has broken the basic principle of how the
> > > swap cache works, we should catch and fix that.
> > >
> > > Signed-off-by: Kairui Song <kasong@...cent.com>
> > > ---
> > >   mm/shmem.c | 28 +++-------------------------
> > >   1 file changed, 3 insertions(+), 25 deletions(-)
> > >
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index b4d39f2a1e0a..e03793cc5169 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -2158,35 +2158,13 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
> > >       /* Swap cache still stores N entries instead of a high-order entry */
> > >       xa_lock_irq(&swap_mapping->i_pages);
> > >       for (i = 0; i < nr_pages; i++) {
> > > -             void *item = xas_load(&xas);
> > > -
> > > -             if (item != old) {
> > > -                     error = -ENOENT;
> > > -                     break;
> > > -             }
> > > -
> > > -             xas_store(&xas, new);
> > > +             WARN_ON_ONCE(xas_store(&xas, new));
> > >               xas_next(&xas);
> > >       }
> > > -     if (!error) {
> > > -             mem_cgroup_replace_folio(old, new);
> > > -             shmem_update_stats(new, nr_pages);
> > > -             shmem_update_stats(old, -nr_pages);
> > > -     }
> >
> > It looks like the shmem statistics update was mistakenly deleted?
>
> Ah, you are right, I'll need to add it back. I somehow misread this as
> the error handling path. Need to add it back just drop the !error
> check.

+1, I will wait for your next version then. Otherwise the patch looks
fine to me.

Chris