Re: [PATCH v2 1/2] mm/huge_memory: do not touch frozen folios in deferred_split_isolate()

[Lance Yang]

On 2026/8/31 17:15, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
>
> 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().
>
> Nothing is lost by skipping.  Everything that frees a queued folio
> unqueues it first, and folio_unqueue_deferred_split() clears
> PG_partially_mapped and brings MTHP_STAT_NR_ANON_PARTIALLY_MAPPED down
> on the way:
>
>      __folio_put(), folios_put_refs()   mm/folio.c
>      __folio_migrate_mapping()          mm/migrate.c
>      shrink_folio_list()                mm/vmscan.c
>
> __folio_freeze_and_split_unmapped() does the same by hand, under the
> list_lru lock it holds across the freeze.  A freeze that ends in
> folio_ref_unfreeze() leaves a folio that is still partially mapped and
> still belongs on the queue.
>
> Fixes: 8422acdc97ed ("mm: introduce a pageflag for partially mapped folios")
> Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@xxxxxxxxx/
> Assisted-by: Claude-Code:claude-opus-5
> Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---

LGTM.

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>