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Tue, 07 Jul 2026 05:59:52 -0700 (PDT) From: Wandun Chen To: vbabka@kernel.org, david@kernel.org, rostedt@goodmis.org, mhiramat@kernel.org, Alexander.Krabler@kuka.com, hughd@google.com, fvdl@google.com, bigeasy@linutronix.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Cc: akpm@linux-foundation.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, ljs@kernel.org, riel@surriel.com, liam@infradead.org, harry@kernel.org, jannh@google.com, lance.yang@linux.dev, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, pfalcato@suse.de Subject: [PATCH v2 1/4] mm/migrate: do not migrate folios mapped into VM_LOCKED VMAs under compaction Date: Tue, 7 Jul 2026 20:59:22 +0800 Message-ID: <20260707125925.3725177-2-chenwandun1@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260707125925.3725177-1-chenwandun1@gmail.com> References: <20260707125925.3725177-1-chenwandun1@gmail.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Wandun Chen When compact_unevictable_allowed=0, unevictable pages should not be migrated. However, mlock_folio_batch in the mlock[all] syscall introduces a race, mlock_folio() sets PG_mlocked immediately but defers PG_unevictable to mlock_folio_batch(), causing pages that are about to become unevictable to be migrated, which violates the intent of compact_unevictable_allowed, and causes spike latency in RT kernels [1]. In order to fix this, migration is forbidden for pages mapped into VMAs marked with VM_LOCKED. In addition, two early-return paths are introduced, filter out mlocked pages, return early to avoid unnecessary operations. Fixes: 90d07210ab55 ("mm: mlock: use folios and a folio batch internally") Reported-by: Alexander Krabler Closes: https://lore.kernel.org/all/DU0PR01MB10385345F7153F334100981888259A@DU0PR01MB10385.eurprd01.prod.exchangelabs.com/ [1] Suggested-by: Vlastimil Babka Signed-off-by: Wandun Chen Link: https://lore.kernel.org/linux-rt-users/33275585-f2db-4779-89f0-3ae24b455a67@suse.cz/#t --- include/linux/compaction.h | 6 ++++++ include/linux/rmap.h | 3 +++ mm/compaction.c | 8 +++++++- mm/migrate.c | 23 +++++++++++++++++++---- mm/rmap.c | 12 +++++++++--- 5 files changed, 44 insertions(+), 8 deletions(-) diff --git a/include/linux/compaction.h b/include/linux/compaction.h index f29ef0653546..04e60f65b976 100644 --- a/include/linux/compaction.h +++ b/include/linux/compaction.h @@ -106,6 +106,7 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order, extern void __meminit kcompactd_run(int nid); extern void __meminit kcompactd_stop(int nid); extern void wakeup_kcompactd(pg_data_t *pgdat, int order, int highest_zoneidx); +extern bool compaction_allow_unevictable(void); #else static inline void reset_isolation_suitable(pg_data_t *pgdat) @@ -131,6 +132,11 @@ static inline void wakeup_kcompactd(pg_data_t *pgdat, { } +static inline bool compaction_allow_unevictable(void) +{ + return true; +} + #endif /* CONFIG_COMPACTION */ struct node; diff --git a/include/linux/rmap.h b/include/linux/rmap.h index 8dc0871e5f00..359c7426b6b9 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -102,6 +102,9 @@ enum ttu_flags { * do a final flush if necessary */ TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock: * caller holds it */ + TTU_RESPECT_MLOCK = 0x100,/* leave VM_LOCKED vmas mapped instead + * of installing a migration entry + */ }; #ifdef CONFIG_MMU diff --git a/mm/compaction.c b/mm/compaction.c index f08765ade014..5d256930e389 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1116,7 +1116,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, is_unevictable = folio_test_unevictable(folio); /* Compaction might skip unevictable pages but CMA takes them */ - if (!(mode & ISOLATE_UNEVICTABLE) && is_unevictable) + if (!(mode & ISOLATE_UNEVICTABLE) && + (is_unevictable || folio_test_mlocked(folio))) goto isolate_fail_put; /* @@ -1898,6 +1899,11 @@ typedef enum { * compactable pages. */ static int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNEVICTABLE_DEFAULT; + +bool compaction_allow_unevictable(void) +{ + return sysctl_compact_unevictable_allowed; +} /* * Tunable for proactive compaction. It determines how * aggressively the kernel should compact memory in the diff --git a/mm/migrate.c b/mm/migrate.c index a786549551e3..3a15eb13e82b 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -1202,7 +1202,7 @@ static void migrate_folio_done(struct folio *src, static int migrate_folio_unmap(new_folio_t get_new_folio, free_folio_t put_new_folio, unsigned long private, struct folio *src, struct folio **dstp, enum migrate_mode mode, - struct list_head *ret) + struct list_head *ret, enum migrate_reason reason) { struct folio *dst; int rc = -EAGAIN; @@ -1210,6 +1210,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, struct anon_vma *anon_vma = NULL; bool locked = false; bool dst_locked = false; + enum ttu_flags ttu = 0; dst = get_new_folio(src, private); if (!dst) @@ -1249,9 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, folio_lock(src); } locked = true; - if (folio_test_mlocked(src)) + if (folio_test_mlocked(src)) { old_folio_state |= FOLIO_WAS_MLOCKED; + if (reason == MR_COMPACTION && !compaction_allow_unevictable()) { + rc = -EBUSY; + goto out; + } + } + if (folio_test_writeback(src)) { /* * Only in the case of a full synchronous migration is it @@ -1324,7 +1331,14 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, /* Establish migration ptes */ VM_BUG_ON_FOLIO(folio_test_anon(src) && !folio_test_ksm(src) && !anon_vma, src); - try_to_migrate(src, mode == MIGRATE_ASYNC ? TTU_BATCH_FLUSH : 0); + + if (mode == MIGRATE_ASYNC) + ttu |= TTU_BATCH_FLUSH; + + if (reason == MR_COMPACTION && !compaction_allow_unevictable()) + ttu |= TTU_RESPECT_MLOCK; + + try_to_migrate(src, ttu); old_folio_state |= FOLIO_WAS_MAPPED; } @@ -1905,7 +1919,8 @@ static int migrate_pages_batch(struct list_head *from, } rc = migrate_folio_unmap(get_new_folio, put_new_folio, - private, folio, &dst, mode, ret_folios); + private, folio, &dst, mode, ret_folios, + reason); /* * The rules are: * 0: folio will be put on unmap_folios list, diff --git a/mm/rmap.c b/mm/rmap.c index 0fb7a1b82cf3..3cb7f6337d38 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2420,6 +2420,9 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, unsigned long pfn; unsigned long hsz = 0; + if ((flags & TTU_RESPECT_MLOCK) && (vma->vm_flags & VM_LOCKED)) + return false; + /* * When racing against e.g. zap_pte_range() on another cpu, * in between its ptep_get_and_clear_full() and folio_remove_rmap_*(), @@ -2741,11 +2744,14 @@ void try_to_migrate(struct folio *folio, enum ttu_flags flags) }; /* - * Migration always ignores mlock and only supports TTU_RMAP_LOCKED and - * TTU_SPLIT_HUGE_PMD, TTU_SYNC, and TTU_BATCH_FLUSH flags. + * Migration normally ignores mlock, but TTU_RESPECT_MLOCK asks it to + * leave folios mapped into VM_LOCKED vmas alone. Only TTU_RMAP_LOCKED, + * TTU_SPLIT_HUGE_PMD, TTU_SYNC, TTU_BATCH_FLUSH and TTU_RESPECT_MLOCK + * are supported. */ if (WARN_ON_ONCE(flags & ~(TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD | - TTU_SYNC | TTU_BATCH_FLUSH))) + TTU_SYNC | TTU_BATCH_FLUSH | + TTU_RESPECT_MLOCK))) return; if (folio_is_zone_device(folio) && -- 2.43.0