Re: [PATCH] mm/swap: fix swap_cache_get_folio() returning a folio that left the swap cache

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 15 Oct 2026 14:05:27 +0800 Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> swap_cache_get_folio() reads the folio from the swap table and takes a
> reference with folio_try_get(), without any lock. If the folio is
> removed from the swap cache and freed in between, and its memory gets
> reused, folio_try_get() still succeeds and we return a page that has
> nothing to do with the swap entry:
> 
>   CPU 0                                  CPU 1
>   do_swap_page()
>     swap_cache_get_folio()
>       swp_tb = swap_table_get()
>       folio = swp_tb_to_folio(swp_tb)
>                                          madvise(MADV_DONTNEED)
>                                            zap_nonpresent_ptes()
>                                              swap_put_entries_direct()
>                                                swap_put_entries_cluster()
>                                                  __try_to_reclaim_swap()
>                                                    swap_cache_del_folio()
>                                                    folio_put()   <- freed
>                                          page reused, e.g. as an anon
>                                          folio of another task
>       folio_try_get(folio)               <- succeeds
>       return folio
>     swap_update_readahead()              <- may clear PG_readahead on it
>     folio_lock_or_retry()                <- locks it
>     folio_matches_swap_entry() fails, unlock and put
> 
> filemap_get_entry() does the same lockless lookup for the page cache,
> and after folio_try_get() it rechecks the xarray, because the folio may
> have been freed and reused in between. swap_cache_get_folio() lost
> that recheck when the swap cache moved to the swap table.
> 
> So CPU 0 writes to a folio it doesn't own: it may clear PG_readahead and
> takes the folio lock of someone else's page, before
> folio_matches_swap_entry() finds out the folio is not ours.
> 
> Recheck the slot after folio_try_get() and retry if it no longer holds
> this folio.

Thanks.

As always, when fixing a bug please fully describe the worst-case
userspace-visible runtime effects of that bug.

I asked $LLM for this and was told

: When the race occurs, a swap fault can acquire a reference to a
: physical page that has been freed and reused for an unrelated purpose,
: potentially by another process.  The fault handler may then clear that
: page's PG_readahead/PG_reclaim flag, wait on or acquire its folio lock,
: and potentially attempt to remove it from an unrelated swap cache.
: 
: These operations can interfere with unrelated workloads and may cause
: unexpected stalls, impaired reclaim or writeback, and potentially
: system hangs.  The subsequent swap-entry validation normally prevents
: the wrong folio from being mapped into the faulting process, and
: neither data corruption nor a kernel crash has been demonstrated.

Do you agree with this?  OK with me adding this to your changelog?

> Fixes: 8578e0c00dcf ("mm, swap: use the swap table for the swap cache and switch API")

Given the above, I suggest adding cc:stable.

Do you and maintainers agree?