Message-ID: <CAMgjq7DPgUNT41pS+591UUR2TfR5pCrABD8y6CKWF-ENpPBN0g@mail.gmail.com>
Date: Mon, 8 Sep 2025 17:28:25 +0800
From: Kairui Song <ryncsn@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.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>, 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 v2 08/15] mm/shmem, swap: remove redundant error handling
 for replacing folio

On Mon, Sep 8, 2025 at 2:04 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2025/9/6 03:13, 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>
> > Reviewed-by: David Hildenbrand <david@...hat.com>
> > ---
> >   mm/shmem.c | 42 ++++++++++++------------------------------
> >   1 file changed, 12 insertions(+), 30 deletions(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 4e27e8e5da3b..cc6a0007c7a6 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1698,13 +1698,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
> >               }
> >
> >               /*
> > -              * The delete_from_swap_cache() below could be left for
> > +              * The swap_cache_del_folio() below could be left for
> >                * shrink_folio_list()'s folio_free_swap() to dispose of;
> >                * but I'm a little nervous about letting this folio out of
> >                * shmem_writeout() in a hybrid half-tmpfs-half-swap state
> >                * e.g. folio_mapping(folio) might give an unexpected answer.
> >                */
> > -             delete_from_swap_cache(folio);
> > +             swap_cache_del_folio(folio);
> >               goto redirty;
> >       }
>
> You should reorganize your patch set, as the swap_cache_del_folio()
> function is introduced in patch 9.
>
> >       if (nr_pages > 1)
> > @@ -2082,7 +2082,7 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
> >       new->swap = entry;
> >
> >       memcg1_swapin(entry, nr_pages);
> > -     shadow = get_shadow_from_swap_cache(entry);
> > +     shadow = swap_cache_get_shadow(entry);
>
> Ditto.
>
> >       if (shadow)
> >               workingset_refault(new, shadow);
> >       folio_add_lru(new);
> > @@ -2158,35 +2158,17 @@ 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);
> > -     }
> >       xa_unlock_irq(&swap_mapping->i_pages);
> >
> > -     if (unlikely(error)) {
> > -             /*
> > -              * Is this possible?  I think not, now that our callers
> > -              * check both the swapcache flag and folio->private
> > -              * after getting the folio lock; but be defensive.
> > -              * Reverse old to newpage for clear and free.
> > -              */
> > -             old = new;
> > -     } else {
> > -             folio_add_lru(new);
> > -             *foliop = new;
> > -     }
> > +     mem_cgroup_replace_folio(old, new);
> > +     shmem_update_stats(new, nr_pages);
> > +     shmem_update_stats(old, -nr_pages);
> > +
> > +     folio_add_lru(new);
> > +     *foliop = new;
> >
> >       folio_clear_swapcache(old);
> >       old->private = NULL;
> > @@ -2220,7 +2202,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
> >       nr_pages = folio_nr_pages(folio);
> >       folio_wait_writeback(folio);
> >       if (!skip_swapcache)
> > -             delete_from_swap_cache(folio);
> > +             swap_cache_del_folio(folio);
> >       /*
> >        * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
> >        * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
> > @@ -2459,7 +2441,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> >               folio->swap.val = 0;
> >               swapcache_clear(si, swap, nr_pages);
> >       } else {
> > -             delete_from_swap_cache(folio);
> > +             swap_cache_del_folio(folio);

Oh you are right, or I should keep the delete_from_swap_cache here.

Let me just rebase and move this patch later then. Thanks!

> >       }
> >       folio_mark_dirty(folio);
> >       swap_free_nr(swap, nr_pages);
>
>

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