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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 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 X-Rspam-User: X-Stat-Signature: tqjrpghdjagtnam8bos13pq5ejomjngh X-Rspamd-Queue-Id: 9E251140007 X-Rspamd-Server: rspam09 X-HE-Tag: 1775069425-331808 X-HE-Meta: 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 UuGzITLS 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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