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Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Wed, 28 Oct 2026 09:46:00 -0400 Message-Id: Subject: Re: [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free Cc: "teawater" To: "Dev Jain" , "Hui Zhu" , "Andrew Morton" , "Vlastimil Babka" , "Suren Baghdasaryan" , "Michal Hocko" , "Brendan Jackman" , "Johannes Weiner" , , From: "Zi Yan" X-Mailer: aerc 0.22.0 References: <20261028090747.68086-1-hui.zhu@linux.dev> <548d9950-cc77-4c1b-b78a-34e304adb989@arm.com> In-Reply-To: <548d9950-cc77-4c1b-b78a-34e304adb989@arm.com> X-ClientProxiedBy: SA0PR11CA0199.namprd11.prod.outlook.com (2603:10b6:806:1bc::24) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) 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 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|SN7PR12MB7153:EE_ 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uRJrFf59yuXIRCWIzyz3w5XG1jQ/c6l6poqXhEUxG/L4njF1VHElAJroxmZlYcqB X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN7PR12MB7153 On Wed Oct 28, 2026 at 6:30 AM EDT, Dev Jain wrote: > > > On 28/10/26 2:37 pm, Hui Zhu wrote: >> From: teawater >>=20 >> KCSAN reports a data-race on nr_free which also exists on mainline: >>=20 >> BUG: KCSAN: data-race in fast_isolate_freepages+0x284/0x1550 mm/compac= tion.c:1589 >>=20 >> 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 >>=20 >> value changed: 0x00000000000002f4 -> 0x00000000000002f5 >>=20 >> 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 ha= s >> no free pages. page_alloc.o is excluded from KCSAN, so the writer is no= t >> instrumented and the report is of unknown origin. >>=20 >> The race is benign: a stale zero only skips the order for this round, an= d >> 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"). >>=20 >> No functional change intended. >>=20 >> Signed-off-by: teawater > > teawater? What? > > >> --- >> mm/compaction.c | 12 ++++++++++-- >> 1 file changed, 10 insertions(+), 2 deletions(-) >>=20 >> 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 =3D 0; >> unsigned long high_pfn =3D 0; >> =20 >> - 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 fac= t. IIRC, checkpatch.pl requires a comment for each data_race(). The comments need to stay until you send a patch to change checkpatch.pl. > > Apart from that, do we have the same problem with zone_watermark_ok? It a= lso > uses area->nr_free without zone lock. > >> + if (!data_race(area->nr_free)) >> continue; >> =20 >> spin_lock_irqsave(&cc->zone->lock, flags); >> @@ -2031,7 +2035,11 @@ static unsigned long fast_find_migrateblock(struc= t compact_control *cc) >> unsigned long flags; >> struct page *freepage; >> =20 >> - 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; >> =20 >> spin_lock_irqsave(&cc->zone->lock, flags); --=20 Best Regards, Yan, Zi