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 9F68126E711; Wed, 1 Apr 2026 18:50:24 +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=1775069424; cv=none; b=JERdPxKypMnZJBdmRLqv8BhEKNGuX//VNo8YoEPAiRWtpDVeZBi0Co/167B8KjW21z8l7Tc2shfSwsPMXUjsPFCVjMmoCAdi4GNEFKKLxCwQym7bB+KRRk89z7ZUIpxyyJO5zGUgW/p0P9xo05qEqVqrCOPxgME+F53wa3fvUhw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775069424; c=relaxed/simple; bh=yFr+6pN+gSthtYPUMkPOCzx9pN9VnMpe50u0D+mLClI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bMGqLmLHqkXJ7gdRnmEWAj+TJjdlkDEUthrZybcwSaxUBMfzvsp8Uy8AFULawSOGvjOzK9bTJa4Yd764E48T8flOyOPKTze9L81+qnTMaegCwgS8QLJLMLgLso34AO6Mlw7D/CrX7fKfJLVKVtIwYpZjD8VKQhcNMRJ8HRIt+2E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=czA3AYEf; 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="czA3AYEf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EF9DCC4CEF7; Wed, 1 Apr 2026 18:50:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775069424; bh=yFr+6pN+gSthtYPUMkPOCzx9pN9VnMpe50u0D+mLClI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=czA3AYEfwyQu2qQtrMZzAfTt3fC4ZfeWDW/pgS2GYNMXZuj+8+RelkDDMQfreIG3N 9ucAmD4ylt+cctWrnbj2YXaMe24h1JJ9/k1ob1TAJyn4GspIJnwF6yLtYWwfWsHvTZ KUEz8RWcUudc/Ik9LTx/JGQa9E3TCjoef5RK9RsDykN5UlOcWrFed4h6uc3bgiXHPu CQ2nHuWGzvzBy8araAnLDYKiAJ9umbxE24yppx5qf4suOA6UftYEUueaT8l87AEPEP nMYq+DpbccZVugR0RvUZy+IqxuAyrfQWT9IQMhxDF6GgZ1L5Xd2bZFX7baGn/eS8E/ kHegycPmozc/A== Message-ID: Date: Wed, 1 Apr 2026 20:50:16 +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: [PATCH mm-unstable 1/1] mm: fix deferred split queue races during migration To: Usama Arif , Lance Yang Cc: akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, richard.weiyang@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kartikey406@gmail.com, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, stable@vger.kernel.org References: <20260401162855.146945-1-usama.arif@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 18:28, Usama Arif wrote: > On Wed, 1 Apr 2026 21:10:32 +0800 Lance Yang wrote: > >> From: Lance Yang >> >> migrate_folio_move() records the deferred split queue state from src and >> replays it on dst. Replaying it after remove_migration_ptes(src, dst, 0) >> makes dst visible before it is requeued, so a concurrent rmap-removal path >> can mark dst partially mapped and trip the WARN in deferred_split_folio(). >> >> Move the requeue before remove_migration_ptes() so dst is back on the >> deferred split queue before it becomes visible again. >> >> Because migration still holds dst locked at that point, teach >> deferred_split_scan() to requeue a folio when folio_trylock() fails. >> Otherwise a fully mapped underused folio can be dequeued by the shrinker >> and silently lost from split_queue. >> >> Link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085 >> Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue") >> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com >> Closes: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/ >> Cc: >> Suggested-by: David Hildenbrand (Arm) >> Signed-off-by: Lance Yang >> --- >> >> [ Backport note ] >> This patch is a follow-up fix for 8a8ca142a488 ("mm: migrate: requeue >> destination folio on deferred split queue"), which is currently only in >> mm-stable, and should be backported together with it. >> >> Credit for this fix goes to David, thanks! >> >> mm/huge_memory.c | 12 +++++++----- >> mm/migrate.c | 18 +++++++++--------- >> 2 files changed, 16 insertions(+), 14 deletions(-) >> > > > Thanks for the fix! And sorry for introducing the bug in > migrate_folio_move() :) > > So I am happy with the migrate_folio_move() change, it makes sense. > > The goto next if folio is locked in deferred_split_scan() was actually > on purpose. The reasoning was that if the folio is locked, we consider > it as in use by someone and therefore we shouldnt split it. Eventhough > thp_underused() does a zero-filled check, the whole point of the shrinker > was to split THPs that are "not in use", and in my mind, locked folio > is a folio in use. So not sure about that change.. That is a questionable assessment. It's about checking whether folios are *underused* not, if they are used, by whoever in the system (e.g., migration). Just take a look when anonymous folios are actually locked :) So the original locked handling here is just bogus. -- Cheers, David