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b=p/VUuOCJQh8aeB+vE+vB9mKMOgpKWTI/45lZFSeViQQ/saY7J2vnxjuQuKbXkaOrM B6d9+0xhDbnrny/1LU+9H+LX59wV/ULs+IOmsFeD3bEawtE3ZBjLwuSAJZHQxPHHRI kWIxlcoA3rrx92EkkwEZCA1QAg1Wiwf3QVJ2gC3XPkq9oKerSqSM7POUby4lkwsRZU jAhlHnQrD14QPpUz2HUpjumgFakBs6WJQq7qYVvqhRm5OYXkTsG1739SKZL4N5OZSd F8fMqwJBsdta2/H4Z7dDz5SaqzIV9+cQXslzOYNG+zsD+a2JDVWU/NKz/Wmcl7q1Hs BIsjHKfV1pyWA== Message-ID: Date: Wed, 1 Apr 2026 12:10:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: fix stale partially_mapped arg to deferred_split_folio() To: Deepanshu Kartikey , akpm@linux-foundation.org, ziy@nvidia.com, 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 Cc: usama.arif@linux.dev, richard.weiyang@gmail.com, sj@kernel.org, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com, Lance Yang References: <20260401084116.22219-1-kartikey406@gmail.com> 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-Rspamd-Queue-Id: 19E7F100005 X-Stat-Signature: 8nscpph89gg3mfyeoawgbcxztpbaqigp X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1775038265-84595 X-HE-Meta: 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 opyxto5B eYmsGopPokVIgeqtUODb+bIhXBnMqLiEJoh0simvJGVeedHbt8DVUAmNkXVnH7C4/Dn3yPlnbkOI2eODeQ6H0zDbw/EGhADqAd6TLXlj8yt813xTeKULEtQtBirG5XXJ+nZBv033IVhkXw75Xng2ME3UtqohOo1Q9ILa8UdJXaWlkjxM9OuI/qqdtJFCZ54WKNAljhdfjwWG06fx/JZ33tmCg9kcu9c/0g1CnkukDhrAmroxDVNi0KmqBSieqmL9FhUCNtIL2DgRQVuCqiFes8zkHK3NIT0BXyPjQF71nt1RNKkz/zRy9qMwsf4vuAbd3xKmGj6hZnUKVuVBDHrpAUAs1wAvR0NZ6CWTsX4ZDzZOxTLUqDaTTrt4D8c1CcjX6H+/GY8LzkvpsSTa8hz2/ULUH3g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/1/26 10:41, Deepanshu Kartikey wrote: > In migrate_folio_move(), src_partially_mapped is sampled from the source > folio before move_to_new_folio() is called: > > if (folio_order(src) > 1 && > !data_race(list_empty(&src->_deferred_list))) { > src_deferred_split = true; > src_partially_mapped = folio_test_partially_mapped(src); > } > > A concurrent thread can unmap pages from the source folio between this > read and the actual migration, making the sampled value stale. Trying to make sense of this. In migrate_folio_move() don't we have the folio completely unmapped because there are only migration entries referencing the folio? See migrate_folio_unmap(), where we check !folio_mapped(). Why should we suddenly have mapped folio here? Something is off. Unmapping a migration entry will not involve rmap code and not mess with the partially-mapped flag. -- Cheers, David