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Thu, 27 Aug 2026 16:38:45 +0000 X-Mizu-Trace-ID: 5f895ce9d536e3b4 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, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kiryl Shutsemau (Meta)" Subject: Re: [PATCH 1/5] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() Date: Thu, 27 Aug 2026 09:38:37 -0700 Message-ID: <20260827163838.1813081-1-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260826162101.1314941-2-kirill@shutemov.name> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: wqxgjd74uu4jko9qnd7kfe1wrmp8aio4 X-Rspamd-Queue-Id: 947A540002 X-Rspamd-Server: rspam06 X-HE-Tag: 1787848726-499594 X-HE-Meta: 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 H2mQ7EaU y9bkJIQjSzq5zPRgQ20CqZHEziI6QjKpaG10G3AA0kSGIn5/P2bZZ+caW++BtlYAkelLKBomTpzA/zDFqN55GfjSh/XUVVrbFd0UL5TEZ/x1VwS2lmZnGpKMdtR7QcowuR3BEictgycz01z7cBVxfLjPdf3WiphaOgiljwRXEnhzrGmzsbrdPdD4V0in15U85gJuMtcOCQNYaZ5SQh3S57KngKHW5uk4D8WNQICb/Fq4kt2TnUAEL20Gcdo4kEcleBETstoqS84kLGhGugd8Kln8huGNtXw6x9u2bTXFkAPxGMHBSdhf7hCqeFls1MVKk9ez0fO/LaWAK/mDXF8J+AwNY7Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 26 Aug 2026 17:20:57 +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(). > > Reported-by: Lance Yang > 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) > --- > 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; I think we might have a problem here for ZONE_DEVICE folios? This assumes the final put always dequeues the folio, but ZONE_DEVICE folios bypass the generic folio_unqueue_deferred_split() path. With memcg disabled, this can leave a recycled folio linked on the deferred-split list? Should we dequeue folios in free_zone_device_folio()? > > - /* > - * 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; > } > > -- > 2.54.0 > >