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Subject: Re: [PATCH 1/5] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() Cc: , , , , , , , , , , , , , , , "Kiryl Shutsemau (Meta)" To: "Usama Arif" , "Kiryl Shutsemau" X-Mailer: aerc 0.22.0 References: <20260826162101.1314941-2-kirill@shutemov.name> <20260827163838.1813081-1-usama.arif@linux.dev> In-Reply-To: <20260827163838.1813081-1-usama.arif@linux.dev> X-ClientProxiedBy: CYXP220CA0009.NAMP220.PROD.OUTLOOK.COM (2603:10b6:930:ee::17) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|SN7PR12MB7977:EE_ X-MS-Office365-Filtering-Correlation-Id: 3b5d61b2-c48e-46e8-e518-08df04a7b424 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|23010399003|1800799024|7416014|376014|10067099003|22082099003|18002099003|11063799006|4143699003|5023799004|56012099006; X-Microsoft-Antispam-Message-Info: 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hUhsCA3ztEdcaPCf+6yPDaZNxhDtP3kSisTIFDpt8BjH+6USu0XC/Ua9CtVNROlEdZhIUA4HGi0H31sVV9ReYBCx3WkWkPwAFaTS7u4Iaw+jrI3be0R5iY9G8Ktwd0D0/Gc11UUJo2TIg8U/ESWTC8OaCh4WkeFeU6yF50utFm5RBCJXT/g8HBOH7ZHv1SA/4MHT0gp5Zm3RJZ2rQM8ppOFTlifk81FG3PLJBO6Dt/QZutryA/bfFZn0YzDgJHwfm71VZ/kEz9o0JqhqQYuUzmZbIsV0xZDQI5aiWcb2bJMGg8Rtggjl3FfDoYOEqz724OqEJqYx/gJgxjooaLc8NdP5Gtac2GNsn+zOLtDpDMgEx2HbE/cfNZKyu+X+oydoqHXElii2LGWog64xk5trBZZ3ssPdayHlfwl55OjnwuO9tSLg386j8rM/FHJvEG7szaH5kk5M54Q8hQwaj5usig1fmlnJ3RfM5cFB5dKtQntFtX50= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu Aug 27, 2026 at 12:38 PM EDT, Usama Arif wrote: > On Wed, 26 Aug 2026 17:20:57 +0100 Kiryl Shutsemau = wrote: > >> From: "Kiryl Shutsemau (Meta)" >>=20 >> deferred_split_isolate() probes each queued folio with folio_try_get(). >> folio_try_get() failure is treated as a lost race with folio_put(): clea= r >> PG_partially_mapped, correct MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, take >> the folio off the queue. >>=20 >> 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(). >>=20 >> 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: >>=20 >> 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() >>=20 >> The folio is still partially mapped, but it is no longer a split candida= te. >> Nothing queues it again until part of it is unmapped once more. >>=20 >> 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(). >>=20 >> Reported-by: Lance Yang >> Link: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linu= x.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(-) >>=20 >> 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(st= ruct list_head *item, >> struct folio *folio =3D container_of(item, struct folio, _deferred_lis= t); >> struct list_head *freeable =3D cb_arg; >> =20 >> - 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? For coherent ZONE_DEVICE folios, yes. IIRC, private ZONE_DEVICE folios are not added to deferred split queue. > > This assumes the final put always dequeues the folio, but ZONE_DEVICE fol= ios > 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()? I think so, before mem_cgroup_uncharge(). But it is a pre-existing issue. We need a separate patch unqueuing folios in free_zone_device_folio() to fix commit a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages"). >> =20 >> - /* >> - * 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; >> } >> =20 >> --=20 >> 2.54.0 >>=20 >>=20 --=20 Best Regards, Yan, Zi