Re: [PATCH 1/5] mm/huge_memory: do not touch frozen folios in deferred_split_isolate()

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Aug 26, 2026 at 05:20:57PM +0100, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> 
> deferred_split_isolate() probes each queued folio with folio_try_get().
> folio_try_get() failure is treated as a lost race with folio_put(): clear
> PG_partially_mapped, correct MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, take
> the folio off the queue.
> 
> The folio_put() race is the most common case for !folio_try_get(), but
> it is not the only option. Another scenario is folio_ref_freeze().
> 
> A zero refcount in such cases does not mean the folio is going away.  It
> means "don't touch me" and current deferred_split_isolate() doesn't
> respect it. It can lead to unqueueing folios from the deferred list for
> no reason:
> 
>     CPU 0                            CPU 1
>     ---------------------------      ------------------------------
>     freeze a mapped folio            deferred_split_scan()
>       folio_ref_freeze()               folio_try_get() fails
>                                        folio_clear_partially_mapped()
>                                        NR_ANON_PARTIALLY_MAPPED--
>                                        folio off the queue
>     give up, put it back
>       folio_ref_unfreeze()
> 
> The folio is still partially mapped, but it is no longer a split candidate.
> Nothing queues it again until part of it is unmapped once more.
> 
> Skip the folio instead: whoever freezes the folio, owns it and owner is
> responsible for its fate. It also covers the folio_put() case:
> __folio_put() unqueues the folio via folio_unqueue_deferred_split().
> 
> Reported-by: Lance Yang <lance.yang@linux.dev>
> Link: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linux.dev/
> Assisted-by: Claude-Code:claude-opus-5
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>