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b=C87ht2XU1p1ey1swk04IfzogavurvJ5V8TQ8YgI1FffaGA64aro2WYLEj5yHRd36V h2CDWj3+M/DEChZcelqV6WMOYHlDDYvUx+BWgdMywl46ggRJSipBnCpybJVrELs2ZU 7Gz80GC/Q+PXSpWWRz7Gcz2HXbDMAzYMP7xVwaFFaV//fp2CFdOrl23NnKB2T4MUPG w0+CtylbAGSFGpiYQzmD739xdKyVIJrH7rMwCpzwBO7teXqRDdQ4gt7Sb7dobh7RP3 CYucX69y+K7/j64jx8hRw9l4SLKZnr6Av+XAqi4ZoulG4M7TAPqA+pBGbNDw/ex6XW ++rjEazkEKfdQ== Message-ID: Date: Wed, 1 Apr 2026 12:15:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: fix stale partially_mapped arg to deferred_split_folio() From: "David Hildenbrand (Arm)" 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> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8297F2000A X-Stat-Signature: 464u4xxzwroiir7z3pjgu6s74kx4ggdn X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1775038540-63393 X-HE-Meta: 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 Var6gFaq jb+gI69BhH8+kAhaadgWqHzanS+es1i7oLsNF8m/9hGNgh7VOi2LJg4viu6ULyyA/EgofUXQ88Pfcu8dS3otiJ93ko2At7jrM1TJrqyTL+ap+opuefgghBIEmNRp35atDhXPCXUP6VNE1kJtzcQpY3+FG1ldC8hQ44hZ6fAp/nW3jtTEWa5YVUIWxqKGtvOabUtzeYJhlq3p3+1GtP1lhYdgIYJX1xcNDrc0KMu3HqhaG4uEVC8rhofC0oIp8Zed9AaokgikvD9pMxeZwv9lXg5N5QrYRDFyQaSqBImTHGkjphKZ2lpncTCniLqm8zj+Mj/W04HkpXMwMghUL0osqbBh1jEVLuoZ3cbKqPvEk2BExIoXptIzhvOV9z1gmZxgvE3pJhm906pwPJ8l7cT4X2S8TavpreExXd+v7IUCdx32J88s= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/1/26 12:10, David Hildenbrand (Arm) wrote: > 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. > Okay, concluding that the above reasoning is all wrong. Let's discuss it with Lance's proposal, which makes more sense. https://lore.kernel.org/r/20260401085932.20945-1-lance.yang@linux.dev -- Cheers, David