From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 53A9740711A for ; Wed, 1 Apr 2026 11:00:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775041220; cv=none; b=LOCfT4l4Nr0/+Q4f5evfs1FkYmQ5Zan3YiEObftGV3aLgbjJffoQfITtP7rSWafh6l5yHnlzVCdIOVCt8cZpdv2ocRgR4EA1QJA5l3OCq/2ILboTOekHolX6JItAEYtwoE26tSYif/OVnAVJXrmoVDG/bbfiN53cmvGGT72xKWE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775041220; c=relaxed/simple; bh=Sn0m1q1+j6lDe7jogLiwV6iavRQIFV9BO8Doky+kRR0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=l66jlwmRHCVegcoQUEmMqXxhzMkYXirkCe158ga1q14Um0e1WNWd9zLdA5O17+GGG/AMZf7uyo3LhRu8jWWGaz9fIdhVs4szT46NL74oYFFTPuaD1bCXL7jMMe3pCMyk8TEq+eiyN18kliWAjbbM6gnMkecE0k7g6iCrjvl8IIU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Lq/TXV9t; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Lq/TXV9t" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DB629C4CEF7; Wed, 1 Apr 2026 11:00:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775041219; bh=Sn0m1q1+j6lDe7jogLiwV6iavRQIFV9BO8Doky+kRR0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Lq/TXV9t+5cmUeGofFuxuXFePuYCo9Pd4hVefeEaXq/h4nd0BIR9v4eRmMhWWtbae XVj2Rclj7WYyU8XDc+dVatQH48JTG+7Nz9VJdAi5W99kditCPzafa2fbpZ4TwwzpsR Ic6JZMa14nT8TaKAHpVZgIkYAIi58Eje9Oh7Ff3oTCo6W+9LGr8Ip7xi23hIPsfw/m T0Uqma/A2vgvD0cLtZs3unjPZHbE87WFfkmZxNB+Y0/SGt3M8XRFIj+ST7vewisAvF mMMZEF1NaeJJ1oOuCld7OhBQUUzU0GEgWpHyTGeZQyx0MiN+ek2qQG6ppcvDWPaaMl HleGU5pyi/Q0A== Message-ID: Date: Wed, 1 Apr 2026 13:00:13 +0200 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: [syzbot] [mm?] WARNING in deferred_split_folio To: Lance Yang , kartikey406@gmail.com Cc: usama.arif@linux.dev, Liam.Howlett@oracle.com, ziy@nvidia.com, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, akpm@linux-foundation.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, npache@redhat.com, ryan.roberts@arm.com, syzkaller-bugs@googlegroups.com References: <27d742c6-631a-4878-9c44-bf49bcce9510@kernel.org> <20260401105305.94886-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/1/26 12:53, Lance Yang wrote: > > +Cc Deepanshu > > On Wed, Apr 01, 2026 at 12:16:43PM +0200, David Hildenbrand (Arm) wrote: >> On 4/1/26 10:59, Lance Yang wrote: >>> >>> >from another sharer can then remove some of those mappings and reach >>> >>> Perhaps the WARN is simply too strict there :) >>> >>> Migration already holds the folio lock on dst, while the competing >>> rmap-removal path runs under the page-table lock. So once >>> remove_migration_ptes(src, dst, 0) makes dst visible again, this race >>> looks hard to avoid. >>> >>> So maybe the simplest fix is just to drop the WARN in the >>> !partially_mapped path: >>> >>> ---8<--- >>> Subject: [PATCH 1/1] mm/thp: avoid false warning in deferred_split_folio() >>> >>> From: Lance Yang >>> >>> migrate_folio_move() snapshots src_partially_mapped from src before >>> migration and later requeues dst after remove_migration_ptes(src, dst, 0). >>> >>> Once dst is visible again, a competing rmap-removal path can legally set >>> PG_partially_mapped before the migration path reaches >>> deferred_split_folio(dst, src_partially_mapped). >>> >>> Migration already holds the folio lock on dst, while the competing >>> rmap-removal path runs under the page-table lock. So once >>> remove_migration_ptes(src, dst, 0) makes dst visible again, this race >>> looks hard to avoid. >>> >>> So just drop the WARN in the !partially_mapped path and preserve an >>> already-set PG_partially_mapped bit. >>> >>> Link: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/ >>> Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue") >>> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com >>> Signed-off-by: Lance Yang >>> --- >>> mm/huge_memory.c | 3 --- >>> 1 file changed, 3 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 745eb3d0d4a7..8ea8e293dc7c 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -4433,9 +4433,6 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped) >>> mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, 1); >>> >>> } >>> - } else { >>> - /* partially mapped folios cannot become non-partially mapped */ >>> - VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio); >>> } >> >> Can't we simply move the setting before restoring migration ptes? > > Afraid not, it closes the remove_migration_ptes() -> > deferred_split_folio() race, but opens a new one with the shrinker, IIUC > > Once dst is on the deferred split queue, deferred_split_scan() can > pick it up immediately. The shrinker unconditionally dequeues every > folio it visits: > > list_del_init(&folio->_deferred_list); /* always */ > > Then for a non-partially-mapped folio, if folio_trylock() fails > (dst is still locked by migration), it falls through to: > > next: > if (did_split || !folio_test_partially_mapped(folio)) > continue; /* not requeued, dst silently lost */ > > so it is *not* requeued. How is that different to the shrinker just trying to lock the folio before we unlock it and failing? The race already exists? To sort out that race a trylock must not result in the folio getting discarded. diff --git a/mm/huge_memory.c b/mm/huge_memory.c index ff9a42abd1b6..521989517cd1 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -4558,7 +4558,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, goto next; } if (!folio_trylock(folio)) - goto next; + goto requeue: if (!split_folio(folio)) { did_split = true; if (underused) @@ -4569,6 +4569,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink, next: if (did_split || !folio_test_partially_mapped(folio)) continue; +requeue: /* * Only add back to the queue if folio is partially mapped. * If thp_underused returns false, or if split_folio fails -- Cheers, David