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

[Kairui Song <ryncsn@gmail.com>]

On Mon, Aug 25, 2025 at 11:09 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/8/23 03:20, Kairui Song wrote:
> > From: Kairui Song <kasong@tencent.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@tencent.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.

Thanks for the review.

>
> ( Continue to understand the whole series:) )
>
> >       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;
> > -     }
> > +     folio_add_lru(new);
> > +     *foliop = new;
> >
> >       folio_clear_swapcache(old);
> >       old->private = NULL;
>
>