Re: [PATCH v2] mm/shmem, swap: fix race of truncate and swap entry split

[Kairui Song <ryncsn@gmail.com>]

On Mon, Jan 19, 2026 at 3:33 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Mon, 19 Jan 2026 00:55:59 +0800 Kairui Song <ryncsn@gmail.com> wrote:
>
> > From: Kairui Song <kasong@tencent.com>
> >
> > I observed random swapoff hangs and kernel panics when stress testing
> > ZSWAP with shmem. After applying this patch, all problems are gone.
> >
> > Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
>
> September 2024.
>
> Seems about right.  A researcher recently found that kernel bugs take two years
> to fix.  https://pebblebed.com/blog/kernel-bugs?ref=itsfoss.com
>
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -962,17 +962,29 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
> >   * being freed).
> >   */
> >  static long shmem_free_swap(struct address_space *mapping,
> > -                         pgoff_t index, void *radswap)
> > +                         pgoff_t index, pgoff_t end, void *radswap)
> >  {
> > -     int order = xa_get_order(&mapping->i_pages, index);
> > -     void *old;
> > +     XA_STATE(xas, &mapping->i_pages, index);
> > +     unsigned int nr_pages = 0;
> > +     pgoff_t base;
> > +     void *entry;
> >
> > -     old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
> > -     if (old != radswap)
> > -             return 0;
> > -     swap_put_entries_direct(radix_to_swp_entry(radswap), 1 << order);
> > +     xas_lock_irq(&xas);
> > +     entry = xas_load(&xas);
> > +     if (entry == radswap) {
> > +             nr_pages = 1 << xas_get_order(&xas);
> > +             base = round_down(xas.xa_index, nr_pages);
> > +             if (base < index || base + nr_pages - 1 > end)
> > +                     nr_pages = 0;
> > +             else
> > +                     xas_store(&xas, NULL);
> > +     }
> > +     xas_unlock_irq(&xas);
> > +
> > +     if (nr_pages)
> > +             swap_put_entries_direct(radix_to_swp_entry(radswap), nr_pages);
> >
> > -     return 1 << order;
> > +     return nr_pages;
> >  }
> >
>
> What tree was this prepared against?
>
> Both Linus mainline and mm.git have
>
> : static long shmem_free_swap(struct address_space *mapping,
> :                           pgoff_t index, void *radswap)
> : {
> :       int order = xa_get_order(&mapping->i_pages, index);
> :       void *old;
> :
> :       old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
> :       if (old != radswap)
> :               return 0;
> :       free_swap_and_cache_nr(radix_to_swp_entry(radswap), 1 << order);
> :
> :       return 1 << order;
> : }
>
> but that free_swap_and_cache_nr() call is absent from your tree.

Oh, I tested and sent this patch based on mm-unstable, because the bug
was found while I was testing swap table series. This is a 2 year old
existing bug though. Swapoff during high system pressure is not a very
common thing, and maybe mTHP for shmem is currently not very commonly
used either? So maybe that's why no one found this issue.

free_swap_and_cache_nr is renamed to swap_put_entries_direct in
mm-unstable, it's irrelevant to this fix or bug. The rename change was
made here:
https://lore.kernel.org/linux-mm/20251220-swap-table-p2-v5-14-8862a265a033@tencent.com/

Should I resend this patch base on the mainline and rebase that
series? Or should we merge this in mm-unstable first then I can
send seperate fixes for stable?