From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=217.140.110.172 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793256356; b=c2p3yA4ffmBW3mh6u1SJb3Im5cB0k98GaEoMYoLCCuegyiG/717uidoUPm4DMvWviiwS QoZd0GU6MUlaX7aJcsRApT1Vq6zGjEhBc6Y2RK2gmpiOEiQ22PsRbuevI0hUUiOd9bIyP bS55ZAq2w0xS5pxcuzPRvlWH51ZuoTsJpORFcCZBV/MW6T4EQCI/h6NV79fj54AV4S97H DRACDWgZwnfkCqW4Yd2LSUibArbzLzmf1o1M+RDwuEYk0ICasOcWv2rtNDkxL0L7Byd5N ava5gCM7k8epYDu3OF6jyXUxJ8AnZ7AmvTwypvpKbE9bkiXh/sznL2SuxSUVBaSyWNA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793256356; h=DMARC-Filter:Received:Received:DKIM-Signature:Message-ID:Date: MIME-Version:User-Agent:Subject:To:Cc:References:Content-Language: From:In-Reply-To:Content-Type:Content-Transfer-Encoding; bh=1HZdEtGRBinobOm262LWtghp2oUY3RgVs+/v7S0ZtPM=; b=5glZme6ockYXbRTB+8cSFpLBXf0jYrtxzmnVXU672NQOGlaYOWMZR3F9Tc3Yg+PUzG/H vK7NPlWWbBU/7FqD/uXryfTwBduRBPpBX8TsqtYSa3e5iKrOq58iYrNe1mGnxCZ/NhGgt NWUBnI9mVRU0MkYh6iCgp0iWX+bVvbapZ60BrzxhFn8fEV3gR61J3v/G7CrD2yp5sYSFG /tUifofm1OQadOgScKJG4oj0yOFEkQE3oe3GeMeJTFQi6eeZ79JpQ+q+hxFkytGT6O5gu Y1LSXYjVcEUoDfcK3ZQQSGNbkiEFCZrFdTPSiavXjAx19KlmpRpYGANwfJajBfhUURA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=erkwwRoG; arc=none smtp.remote-ip=217.140.110.172 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 7FE12606A31A for ; Thu, 29 Oct 2026 06:45:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 7FE12606A31A Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=erkwwRoG Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 92D5B1682; Wed, 28 Oct 2026 23:45:51 -0700 (PDT) Received: from [10.163.153.30] (unknown [10.163.153.30]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4D8E93F673; Wed, 28 Oct 2026 23:45:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1793256354; bh=wEceBPDAoI7jkb3PQGl1/s7MQmUjV8j5qCga2ORy3jo=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=erkwwRoG6+fk1Y3jDeJ13nGzcRLsYy0R9OCVLIYLTULcKkEtbklrmnBpAtpbme0Mp yc1Ho19txmvw15SBggNRsxjjRXm0L55bGaaQLANavxQ2otIesqDLYAA7aE0p+al4JJ L0F9uugIDa/NKe4ON8P8/Xt9RLydxLcoTZ9T4d4Q= Message-ID: Date: Thu, 29 Oct 2026 12:15:47 +0530 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: Mozilla Thunderbird Subject: Re: [PATCH] mm/compaction: annotate data race for zone->free_area[order].nr_free To: Hui Zhu , 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> Content-Language: en-US From: Dev Jain In-Reply-To: <20261028090747.68086-1-hui.zhu@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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 > --- Thanks. With the SOB fixed: Reviewed-by: Dev Jain Is a Fixes: tag required? I think stable backport would be overkill but Fixes makes sense as this leads to a reproducible stack trace? > 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. > + */ > + 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);