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Mon, 29 Jun 2026 02:07:48 -0700 (PDT) Message-ID: <05d7369e-341c-49f4-ae13-df3d0ad930d7@gmail.com> Date: Mon, 29 Jun 2026 17:07:36 +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: Alexander Krabler , "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" , 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: 8bit On 6/26/26 21:42, Alexander Krabler wrote: > On 6/26/26 11:38, Wandun wrote: >> On 6/26/26 16:45, Alexander Krabler wrote: >>> However, we were not able to reproduce the actual race >>> (mlockall() process waiting on a migration PTE), >>> not in the past, not now. Might be hard to trigger that race. >> >> Not hard to trigger that case, I added a debug message, such as below, >> lots of messages occur in a few second. >> >> diff --cc mm/memory.c >> index ff338c2abe92,ff338c2abe92..6552b3b14f78 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@@ -4768,6 -4768,6 +4768,8 @@@ vm_fault_t do_swap_page(struct vm_faul >> if (softleaf_is_migration(entry)) { >> migration_entry_wait(vma->vm_mm, vmf->pmd, >> vmf->address); >> + if (!strcmp(current->comm, "repro")) >> + pr_err("============== hit ================\n"); >> } else if (softleaf_is_device_exclusive(entry)) { >> vmf->page = softleaf_to_page(entry); >> ret = remove_device_exclusive_entry(vmf); > > I have a kprobe on migration_entry_wait set and logged into a ftrace buffer > (including kernel stacktrace). > Yes, this function is hit, but only inside the mmap-syscall, which is okay, > memory allocation is not realtime-safe. > > repro-2090 [002] d.... 811.129549: frt_migration_entry_wait: (migration_entry_wait+0x0/0x100) > repro-2090 [002] d.... 811.129553: > => migration_entry_wait > => __handle_mm_fault > => handle_mm_fault > => __get_user_pages > => populate_vma_page_range > => __mm_populate > => vm_mmap_pgoff > => ksys_mmap_pgoff > => __arm64_sys_mmap > => el0_svc_common.constprop.0 > => do_el0_svc > => el0_svc > => el0t_64_sync_handler > => el0t_64_sync > > The original race was an instruction abort interrupt out of nothing due > to the migration PTE set by kcompactd. > And these kind of races I see quite often on non mlockall()-processes, > but can't reproduce on memory locked processes. > > Example: > podman-832 [000] d.... 812.447820: frt_migration_entry_wait: (migration_entry_wait+0x0/0x100) > podman-832 [000] d.... 812.447823: > => migration_entry_wait > => __handle_mm_fault > => handle_mm_fault > => do_page_fault > => do_translation_fault > => do_mem_abort > => el0_da > => el0t_64_sync_handler > => el0t_64_sync Hi, Alexander >From the perspective of the root cause, there is no fundamental difference between these two call stacks. I modified the reproduction program, and it can still reproduce the situation of the second call stack (although it doesn't occur as frequently). The complete reproduction program is as follows: #define _GNU_SOURCE #include #include #include #include #include #include #include #define PAGE_SIZE 4096 #define NR_PAGES 10000 static void *worker_fn(void *arg) { int fd = (long)arg; size_t len = NR_PAGES * PAGE_SIZE; while (1) { if (ftruncate(fd, 0) < 0) {} if (ftruncate(fd, len) < 0) {} char *p = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (p == MAP_FAILED) continue; mlockall(MCL_ONFAULT | MCL_FUTURE); for (int i = 0; i < NR_PAGES; i++) { for (int j = 0; j < PAGE_SIZE; j++) { p[i * PAGE_SIZE + j] = 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) { int nproc = sysconf(_SC_NPROCESSORS_ONLN); if (nproc < 1) nproc = 1; int *fds = calloc((size_t)nproc, sizeof(int)); if (!fds) return 1; size_t len = NR_PAGES * PAGE_SIZE; for (int i = 0; i < nproc; i++) { char path[64]; snprintf(path, sizeof(path), "./repro_%d.dat", i); unlink(path); fds[i] = open(path, O_RDWR | O_CREAT, 0600); if (fds[i] < 0) return 1; if (ftruncate(fds[i], len) < 0) return 1; } printf("repro: %d workers, %d pages, Ctrl-C to stop\n", nproc, NR_PAGES); pthread_t compact; pthread_create(&compact, NULL, compact_fn, NULL); pthread_t *threads = calloc((size_t)nproc, sizeof(pthread_t)); for (int i = 0; i < nproc; i++) pthread_create(&threads[i], NULL, worker_fn, (void *)(long)fds[i]); pthread_join(compact, NULL); return 0; } > > Thanks, > Alexander > > -- > > KUKA Deutschland GmbH Board of Directors: Michael Jürgens (Chairman), Johan Naten, Hui Zhang Registered Office: Augsburg HRB 14914 > > This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of contents of this e-mail is strictly forbidden. > > Please consider the environment before printing this e-mail.