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Howlett" , Lorenzo Stoakes , Ryan Roberts , Sasha Levin Subject: [PATCH 6.18.y] mm/huge_memory: do not touch frozen folios in deferred_split_isolate() Date: Tue, 10 Nov 2026 09:13:20 -0500 Message-ID: <20261110141218.1833760-1-sashal@kernel.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261107050843-failed-6.18-5da8ec3262bb@kernel.org> References: <20261107050843-failed-6.18-5da8ec3262bb@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: "Kiryl Shutsemau (Meta)" [ Upstream commit 5da8ec3262bbb40f057f3b0dacc18f37bda06f95 ] Patch series "Fix deferred_split_isolate() and drop the split workaround", v2. deferred_split_isolate() probes each queued folio with folio_try_get(). folio_try_get() failure is treated as a lost race with folio_put(). It leads to wrong results when !folio_try_get() was not caused by folio_put(): for a frozen folio, PG_partially_mapped gets wrongfully cleared and the folio dropped from the queue. It came up in the review of my collapse RFC series: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linux.dev/ The bug is inert in upstream code: - __folio_split() works around it; - __folio_migrate_mapping() freezes a folio it is about to replace; - reclaim freezes only what try_to_unmap() already unmapped. No cc:stable needed. But my collapse rework steps on it, so it is worth fixing. The branch the first patch removes also hid an inert, pre-existing bug in the zone device path: https://lore.kernel.org/all/20260827163838.1813081-1-usama.arif@linux.dev/ The first patch fixes deferred_split_isolate(). The second patch removes the workaround for this deferred_split_isolate() behaviour from __folio_freeze_and_split_unmapped(). Tested in a VM: split_huge_page_test, folio_split_race_test and cow pass. Also ran a test that leaves 16 partially mapped THPs on the deferred split queue and drives thp-deferred_split through debugfs, checking nr_anon_partially_mapped. This patch (of 2): 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. Link: https://lore.kernel.org/20260831091514.1879786-1-kirill@shutemov.name Link: https://lore.kernel.org/20260831091514.1879786-2-kirill@shutemov.name Fixes: 8422acdc97ed ("mm: introduce a pageflag for partially mapped folios") Signed-off-by: Kiryl Shutsemau (Meta) Signed-off-by: Andrew Morton Reported-by: Lance Yang Closes: https://lore.kernel.org/all/20260824131224.73344-1-lance.yang@linux.dev/ Reviewed-by: Zi Yan Reviewed-by: Johannes Weiner Reviewed-by: Lance Yang Acked-by: Usama Arif Reviewed-by: Baolin Wang Acked-by: David Hildenbrand (Arm) Assisted-by: Claude-Code:claude-opus-5 Cc: Balbir Singh Cc: Barry Song Cc: Dev Jain Cc: Hugh Dickins Cc: Kairui Song Cc: Liam R. Howlett Cc: Lorenzo Stoakes Cc: Ryan Roberts [ sashal: adapted deferred_split_isolate() changes to the existing deferred_split_scan() loop because the list_lru conversion is absent ] Signed-off-by: Sasha Levin --- mm/huge_memory.c | 9 ++------- 1 file changed, 2 insertions(+), 7 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 78fecc3ad5700..9f547400366b4 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -4310,14 +4310,9 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, _deferred_list) { if (folio_try_get(folio)) { folio_batch_add(&fbatch, folio); - } else if (folio_test_partially_mapped(folio)) { - /* We lost race with folio_put() */ - folio_clear_partially_mapped(folio); - mod_mthp_stat(folio_order(folio), - MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1); + list_del_init(&folio->_deferred_list); + ds_queue->split_queue_len--; } - list_del_init(&folio->_deferred_list); - ds_queue->split_queue_len--; if (!--sc->nr_to_scan) break; if (!folio_batch_space(&fbatch)) -- 2.53.0