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Mon, 31 Aug 2026 11:23:12 +0000 X-Mizu-Trace-ID: 18fcbf994d3f25d8 X-Migadu-Flow: FLOW_OUT From: Usama Arif To: Kiryl Shutsemau Cc: Usama Arif , akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, hannes@cmpxchg.org, lance.yang@linux.dev, ziy@nvidia.com, kasong@tencent.com, hughd@google.com, baolin.wang@linux.alibaba.com, baohua@kernel.org, liam@infradead.org, nico.pache@linux.dev, dev.jain@arm.com, ryan.roberts@arm.com, balbirs@nvidia.com, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/2] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() Date: Mon, 31 Aug 2026 04:23:03 -0700 Message-ID: <20260831112305.466147-1-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260831091514.1879786-2-kirill@shutemov.name> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 13454C0006 X-Stat-Signature: is4k4o3m58zf1dak71txko8h9soqb77k X-HE-Tag: 1788175393-275509 X-HE-Meta: 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 Xn/y/rnM WmAgmlJA3tEjPS5+1uCqrxtaqitjik/dEekkopUxkY0SDrEbbDpEOPSYKhYmh6C+mDawnnPLSnbf3RIApXMULjL1fcjgOlFduzjUnrATw1I1MdBkWQCRc8pDnqfbjfJ+RqELfy3aonV51aTnmXCg9E4y7J0/9Zs6k3bEB8PvB22mfUhCxXjpKRUJBXw38Il9T6BaLAz9hu+dRLz+YSSRxmkEcVcYVq+uPUZbznyFb2SVRTSMmDCUJ9tNH2SrZh2xhvmTQFWU9xXnShwRB8J5m2p9xmWXN6qbriR41B3AJYxpuf7BR6BdxDJHL6TAujTI4Ek//yXMQfQhEu9QPUKTTagYmgmrQLG7hZkbnngsOrn0FR6kx9kWuztMikGIlvlVE9LLn Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, 31 Aug 2026 10:15:13 +0100 Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" > > 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 > Closes: 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) > Reviewed-by: Zi Yan > Reviewed-by: Johannes Weiner > --- > mm/huge_memory.c | 19 ++++--------------- > 1 file changed, 4 insertions(+), 15 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index ced400f72d43..6281ed993243 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4590,22 +4590,11 @@ static enum lru_status deferred_split_isolate(struct list_head *item, > struct folio *folio = container_of(item, struct folio, _deferred_list); > struct list_head *freeable = cb_arg; > > - if (folio_try_get(folio)) { > - list_lru_isolate_move(lru, item, freeable); > - return LRU_REMOVED; > - } > + /* Lost race to folio_put() or the folio is under folio_ref_freeze() */ > + if (!folio_try_get(folio)) > + return LRU_SKIP; > > - /* > - * We lost race with folio_put(). Read folio state before the > - * isolate: folio_unqueue_deferred_split() checks list_empty() > - * locklessly, so once removed the folio can be freed any time. > - */ > - if (folio_test_partially_mapped(folio)) { > - folio_clear_partially_mapped(folio); > - mod_mthp_stat(folio_order(folio), > - MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); > - } > - list_lru_isolate(lru, item); > + list_lru_isolate_move(lru, item, freeable); > return LRU_REMOVED; > } Acked-by: Usama Arif > > -- > 2.54.0 > >