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Wed, 24 Jun 2026 04:08:15 -0700 (PDT) Message-ID: Date: Wed, 24 Jun 2026 19:08:06 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/3] mm/compaction: skip isolate mlocked folios when compact_unevictable_allowed=0 To: "Vlastimil Babka (SUSE)" , 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, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, david@kernel.org, ljs@kernel.org, liam@infradead.org, rppt@kernel.org, bigeasy@linutronix.de, clrkwllms@kernel.org, Alexander.Krabler@kuka.com, Hugh Dickins References: <20260604023812.3700316-1-chenwandun1@gmail.com> <20260604023812.3700316-2-chenwandun1@gmail.com> <969cb14b-5b8b-48e6-add6-4dd13101dd89@kernel.org> <040788a9-e0d5-478e-bb48-3d22b8b41020@gmail.com> Content-Language: en-US From: Wandun In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/22/26 17:55, Vlastimil Babka (SUSE) wrote: > On 6/18/26 13:43, Wandun wrote: >> >> >> On 6/18/26 02:52, Vlastimil Babka (SUSE) wrote: >>> On 6/4/26 04:38, Wandun Chen wrote: >>>> From: Wandun Chen >>>> >>>> compact_unevictable_allowed is default 0 under PREEMPT_RT, >>>> isolate_migratepages_block() skips folios with PG_unevictable set. >>>> However, mlock_folio() sets PG_mlocked immediately but defers >>>> PG_unevictable to mlock_folio_batch(), result in a folio with >>>> PG_mlocked=1 but PG_unevictable=0. Compaction will isolate such a >>>> folio. >>>> >>>> Fix by checking folio_test_mlocked() together with the existing >>>> folio_test_unevictable() check. >>>> >>>> A similar issue has been reported by Alexander Krabler on a 6.12-rt >>>> aarch64 system. Vlastimil suggested to check the mlocked flag [1]. >>>> >>>> Reported-by: Alexander Krabler >>>> Closes: https://lore.kernel.org/all/DU0PR01MB10385345F7153F334100981888259A@DU0PR01MB10385.eurprd01.prod.exchangelabs.com/ >>>> Suggested-by: Vlastimil Babka >>>> Signed-off-by: Wandun Chen >>>> Link: https://lore.kernel.org/all/33275585-f2db-4779-89f0-3ae24b455a67@suse.cz/ [1] >>> >>> Well in that thread, Hugh doubted my suggestion and then it seems we didn't >>> concluded anything. Did you actually in practice observe the issue that >>> Alexander had, and that this patch fixed it, or is that theoretical? >>> >> Yes, I wrote a test case that can reproduce it in a few second. >> >> The test case contains 3 steps: >> 1. mlockall >> 2. mmap file(2GB) + trigger file write page fault; >> 3. during step 1, trigger compact via /proc/sys/vm/compact_memory >> >> >> My reproduction environment is qemu with 4GB ram, 8 core, aarch64, >> preempt_rt and includes the tracepoint in patch 02. >> After running the reproduction program for a few seconds, the >> following output appears. > > Ah, nice. > >> repro-403 [004] ....1 101.270505: mm_compaction_isolate_folio: pfn=0x71e3a mode=0x0 flags=referenced|uptodate|mlocked >> repro-403 [004] ....1 101.270507: mm_compaction_isolate_folio: pfn=0x71e3b mode=0x0 flags=referenced|uptodate|mlocked >> repro-403 [004] ....1 101.270513: mm_compaction_isolate_folio: pfn=0x71e3c mode=0x0 flags=referenced|uptodate|mlocked >> repro-403 [004] ....1 101.270515: mm_compaction_isolate_folio: pfn=0x71e3d mode=0x0 flags=uptodate|mlocked >> repro-403 [004] ....1 101.270517: mm_compaction_isolate_folio: pfn=0x71e3e mode=0x0 flags=uptodate|mlocked >> repro-403 [004] ....1 101.270520: mm_compaction_isolate_folio: pfn=0x71e3f mode=0x0 flags=uptodate|mlocked >> >> >> Unfortunately, I recently found that there is still a bug in the >> fix patch. Setting mlocked in the mlock_folio function could happen >> even after the page is successfully isolated, so it still cannot >> prevent migration. Because of this, I need to think more about how >> to fix it. >> >> Perhaps we should double-check whether the page is mlocked during >> the actual migration phase. > > So IIUC the isolation+migration might be started between the folio is > allocated, and mlocked? In that case the check during migration could still Yes, in that case it still be racy, it is not a good idea to check page flags. > be racy, and if the page is isolated, it's already bad for the RT process. IIUC, more accurately, the migration entry in the page talbe is real a bad for RT process, because isolate page doesn't modify the page table, so memory access continues as usual, therefore a new idea occur. S1. In the mlock[all] syscall, if mlock_vma_pages_range hit a migration entry, then, it should wait for the migration to complete. S2. During the unmap phase of memory migration, prevent a page from being unmapped if the page's associated vma is markd with VM_LOCKED, similar to how reclaim is disabled for pages in a VM_LOCKED vma(try_to_unmap_one). For a page handled during the mlock[all] syscall: - if migration has been already finished, there is noting to do; - if migration is in progress and the migration etnry is already filled, we wait (S1) - if the page is in-fight, going to be isolated/migrated, S2 prevents the unmap. For a page handled during a page fault: VM_LOCKED is already set on the vma, so S2 guarantees it will not be unmapped, hence no migration entry. Thanks a lot for the detailed feedback, Vlastimil. Best regards, Wandun > > So this would only be a short-term problem after the mlockall, but we don't > have a way for the RT process to know the moment it's all settled, right? Yes, some pages may have been isolated and will do migration. > Probably the proper solution would be for mlock[all]() itself to wait for an > isolated page, and only continue once it knows it can't be isolated anymore. > This might howver would go against some of the folio batching optimizations? > >> What do you think of this best-effort approach? >> >> >> Best regards, >> Wandun >> >> >> >> >> >> The full reproducer is as below: >> >> /* gcc repro.c -o repro -lpthread */ >> >> #define _GNU_SOURCE >> #include >> #include >> #include >> #include >> #include >> #include >> >> #define PAGE_SIZE 4096 >> #define NR_PAGES 32 >> #define FILE_SIZE (2ULL * 1024 * 1024 * 1024) >> >> static void *worker_fn(void *arg) >> { >> int fd = (long)arg; >> size_t len = (size_t)FILE_SIZE; >> char *p = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); >> if (p == MAP_FAILED) >> return NULL; >> >> for (size_t off = 0; off + NR_PAGES * PAGE_SIZE <= len; >> off += NR_PAGES * PAGE_SIZE) { >> for (int i = 0; i < NR_PAGES; i++) >> p[off + i * PAGE_SIZE] = 1; >> usleep(200); >> } >> >> munmap(p, len); >> return NULL; >> } >> >> static void *compact_fn(void *arg) >> { >> (void)arg; >> int fd = open("/proc/sys/vm/compact_memory", O_WRONLY); >> if (fd < 0) >> return NULL; >> >> while (1) { >> if (write(fd, "1", 1) < 0) {} >> usleep(5000); >> } >> } >> >> int main(void) >> { >> mlockall(MCL_CURRENT | MCL_FUTURE); >> >> int fd = open("./repro_largefile.dat", O_RDWR | O_CREAT, 0600); >> if (fd < 0) >> return 1; >> unlink("./repro_largefile.dat"); >> if (ftruncate(fd, (off_t)FILE_SIZE) < 0) >> return 1; >> >> printf("repro_largefile: 1 worker, %d pages/batch, Ctrl-C to stop\n", >> NR_PAGES); >> >> pthread_t compact, worker; >> pthread_create(&compact, NULL, compact_fn, NULL); >> pthread_create(&worker, NULL, worker_fn, (void *)(long)fd); >> >> pthread_join(worker, NULL); >> return 0; >> } >> >>>> --- >>>> mm/compaction.c | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/compaction.c b/mm/compaction.c >>>> index b776f35ad020..7e07b792bcb5 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; >>>> >>>> /* >>> >> >