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bh=XbuiEYOxIfwUc/1yQoSDFfJ/vqmXXN2lOdBbPBDeo00=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793254769; v=1; x=1793859569; b=t0GaDORoxdS6kVlTG2PkR06Y2gIQMt7yWZDAXFsJ7F0rZA7IjYMDoHcHOVnTx3NHpSGlTCP+ 37zPYUuOl991HVgAVy/qNFW36HDpQW42nQ7yyY7gLiLAoJPKcqb+08KmrDGlxr/2HziuNEbaXuL P6FenMZ29S3Km+cP1jfp30i8= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id b10954797ac84e40; Thu, 29 Oct 2026 06:19:29 +0000 X-Mizu-Trace-ID: b10954797ac84e40 X-Migadu-Flow: FLOW_OUT Message-ID: <8fe9539a-1a0b-44a0-aec9-032235e8fee3@linux.dev> Date: Thu, 29 Oct 2026 14:19:17 +0800 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Betterbird (macOS/Intel) Subject: Re: [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free To: Dev Jain , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: teawater References: <20261028090747.68086-1-hui.zhu@linux.dev> <548d9950-cc77-4c1b-b78a-34e304adb989@arm.com> Content-Language: en-US From: Hui Zhu In-Reply-To: <548d9950-cc77-4c1b-b78a-34e304adb989@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2026/10/28 18:30, Dev Jain 写道: > > On 28/10/26 2:37 pm, Hui Zhu wrote: >> From: teawater >> >> KCSAN reports a data-race on nr_free which also exists on mainline: >> >> BUG: KCSAN: data-race in fast_isolate_freepages+0x284/0x1550 mm/compaction.c:1589 >> >> race at unknown origin, with read to 0xffff9894ff3dcb60 of 8 bytes by task 179078 on cpu 12: >> fast_isolate_freepages+0x284/0x1550 mm/compaction.c:1589 >> isolate_freepages+0x52/0x650 mm/compaction.c:1722 >> compaction_alloc_noprof+0x36e/0x3e0 mm/compaction.c:1850 >> compaction_alloc+0x21/0x30 mm/compaction.c:1880 >> migrate_folio_unmap+0x7f/0x820 mm/migrate.c:1177 >> migrate_pages_batch+0x37e/0x1200 mm/migrate.c:1762 >> migrate_pages_sync+0xce/0x560 mm/migrate.c:1928 >> migrate_pages+0x8c6/0xc20 mm/migrate.c:2037 >> compact_zone+0x539/0x10b0 mm/compaction.c:2660 >> compact_zone_order+0x1b8/0x230 mm/compaction.c:2795 >> try_to_compact_pages+0x20b/0x630 mm/compaction.c:2851 >> __alloc_pages_direct_compact+0x98/0x2b0 mm/page_alloc.c:3722 >> __alloc_pages_slowpath.constprop.0+0x452/0xdf0 mm/page_alloc.c:4401 >> __alloc_pages_noprof+0x28a/0x2d0 mm/page_alloc.c:4794 >> alloc_pages_mpol_noprof+0x1b1/0x470 mm/mempolicy.c:2270 >> alloc_pages_noprof+0x7a/0xe0 mm/mempolicy.c:2347 >> io_mem_alloc_compound io_uring/io_uring.c:2848 [inline] >> io_pages_map+0xb9/0x330 io_uring/io_uring.c:2892 >> io_allocate_scq_urings+0x231/0x300 io_uring/io_uring.c:3933 >> io_uring_create+0x29c/0x9c0 io_uring/io_uring.c:4091 >> io_uring_setup+0xb6/0x100 io_uring/io_uring.c:4203 >> __do_sys_io_uring_setup io_uring/io_uring.c:4230 [inline] >> __se_sys_io_uring_setup io_uring/io_uring.c:4224 [inline] >> __x64_sys_io_uring_setup+0x66/0xe0 io_uring/io_uring.c:4224 >> x64_sys_call+0x1816/0x1d90 arch/x86/include/generated/asm/syscalls_64.h:426 >> do_syscall_x64 arch/x86/entry/common.c:51 [inline] >> do_syscall_64+0x70/0x2a0 arch/x86/entry/common.c:81 >> entry_SYSCALL_64_after_hwframe+0x76/0xe0 >> >> value changed: 0x00000000000002f4 -> 0x00000000000002f5 >> >> nr_free is only updated with zone->lock held, but fast_isolate_freepages() >> and fast_find_migrateblock() read it locklessly to skip an order that has >> no free pages. page_alloc.o is excluded from KCSAN, so the writer is not >> instrumented and the report is of unknown origin. >> >> The race is benign: a stale zero only skips the order for this round, and >> a stale non-zero just finds an empty MIGRATE_MOVABLE freelist under the >> lock. Annotate both reads with data_race(), following commit >> af1c31acc853 ("mm/vmstat: annotate data race for zone->free_area[order].nr_free"). >> >> No functional change intended. >> >> Signed-off-by: teawater > teawater? What? Oops! Should be: Signed-off-by: Hui Zhu > > >> --- >> mm/compaction.c | 12 ++++++++++-- >> 1 file changed, 10 insertions(+), 2 deletions(-) >> >> diff --git a/mm/compaction.c b/mm/compaction.c >> index f1b2060eb2016..499ec7ceaa3ed 100644 >> --- a/mm/compaction.c >> +++ b/mm/compaction.c >> @@ -1601,7 +1601,11 @@ static void fast_isolate_freepages(struct compact_control *cc) >> unsigned int order_scanned = 0; >> unsigned long high_pfn = 0; >> >> - if (!area->nr_free) >> + /* >> + * Access to nr_free is lockless as it is only a hint; a stale >> + * value is harmless. Use data_race to avoid KCSAN warning. >> + */ > The comment is noise, the reason of using data_race() is a well-known fact. > > Apart from that, do we have the same problem with zone_watermark_ok? It also > uses area->nr_free without zone lock. Yes, __zone_watermark_ok() has the same lockless read of area->nr_free (mm/page_alloc.c:3646), but it does not need annotating. mm/Makefile sets KCSAN_SANITIZE_page_alloc.o := n, and all writers of nr_free live in that same translation unit (__add_to_free_list() at page_alloc.c:792 and __del_page_from_free_list() at page_alloc.c:843, both static inline). With both sides uninstrumented, no watchpoint is ever installed and KCSAN cannot report it. That is also why the compaction report says "race at unknown origin": the read side in compaction.c is instrumented, the writer in page_alloc.c is not, so KCSAN infers the race without being able to attribute it (kernel/kcsan/core.c:687). af1c31acc853 hit the same thing in vmstat.c. The race is equally benign: a stale zero fails the watermark check and the allocator retries; a stale non-zero passes it, and __rmqueue() re-checks under zone->lock via find_suitable_fallback() (page_alloc.c:2266). So annotating page_alloc.c:3646 would be a documentation-only change to another subsystem, silencing a report that cannot occur. I'd keep this patch scoped to mm/compaction. If the KCSAN exclusion for page_alloc.o is ever lifted, it will need the annotation then -- but that means sweeping the whole file. I also checked the remaining readers of free_area[].nr_free; all are under zone->lock: show_mem.c:378, page_reporting.c:188, page_alloc.c:2266. Best, Hui >> + if (!data_race(area->nr_free)) >> continue; >> >> spin_lock_irqsave(&cc->zone->lock, flags); >> @@ -2031,7 +2035,11 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc) >> unsigned long flags; >> struct page *freepage; >> >> - if (!area->nr_free) >> + /* >> + * Access to nr_free is lockless as it is only a hint; a stale >> + * value is harmless. Use data_race to avoid KCSAN warning. >> + */ >> + if (!data_race(area->nr_free)) >> continue; >> >> spin_lock_irqsave(&cc->zone->lock, flags);