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b=CiavmS911DInJ74DVkXRLrW9SLVwTuAZ4qk6H0/Tn4FYAfwg0DXscwmHSHp3bftH0 QajgYDGFeWru4s99Hjwu0ziahN10DVli0avm8Ifs3F88Q2VeB1wiJTNnks1K5o20Pv WNqrIkNDf7yGZfhmDNZRCyGfz7ZZXuYRaYlCFPCVwKLseu4mwTtf87amLeNMmqZStJ aYV6DFu19BIZ8zXVZHADlw+TmibOeSzBHG1HqQSFaG7/ImLDHvmLnM+8cgce5RaZAA YAXLAMzhcLMrQlvNf3P+fb0THc8/CsXvL6IOEH5TQl6V9BONalTh6/xXxZE2hS7Va8 TrCflNjG2Emkg== Message-ID: Date: Mon, 7 Sep 2026 15:48:19 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() To: Kiryl Shutsemau , akpm@linux-foundation.org, ljs@kernel.org, hannes@cmpxchg.org, usama.arif@linux.dev Cc: 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 References: <20260831091514.1879786-1-kirill@shutemov.name> <20260831091514.1879786-2-kirill@shutemov.name> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: fkk3wzijpkwxqsdtwpxk7xf3bb156sjd X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A649820002 X-Rspam-User: X-HE-Tag: 1788788907-931594 X-HE-Meta: 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 ussmH57c EsGOjpPL1NJcLxV6vJTYlfsp9incMSiOs260Gcv9adXIUcl6yAqhn6XoXR0BWq7BKCFC41Y1tHT3//7/UVu/gY4tAraVmsBYQHZZHwtvW3xY4/qX0N1h2r3W8fZYd8UsTeFxfLJTzqROuEXswEAh2eKhEnKpWp7q3gvgd+ADJnCnjN9wmRP+5NmJ0TF91ViUyAeP7aJbK/k0NqZzwpwHfle7yORlIqxyRsHnmmejH7auRikxax3mIjq31Ij4Y1eoyXg/nZTYDfoevP4EuN51mntmaykKMCDzHxU9sfJyp6op/q+wCFfUJLrA59RFQsND00MaTZPvhIkXewsdVsCFOcBujcw6xLO6iRLspnrTz4CcMrxMkbwE4Eic9WmSjsGGsPXqSSKGu+0hnk/t/LackpyolmlHUmf3MsCvz Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/31/26 11:15, 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 > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David