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Wong" , Carlos Maiolino , Alexander Viro , Jan Kara , Matthew Wilcox , Andrew Morton , Ritesh Harjani , Zhang Yi , Christoph Hellwig , Dave Chinner , Daniel Gomez , Theodore Tso , linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andres Freund Subject: Re: [RFC PATCH v4 12/12] iomap: Handle deadlock due to repeating folios in RWF_WRITETHROUGH Message-ID: References: <5159efdec832e8179e9f689a9292f9bf47d9a0fa.1790596383.git.ojaswin@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5159efdec832e8179e9f689a9292f9bf47d9a0fa.1790596383.git.ojaswin@linux.ibm.com> X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: BABC0180004 X-Rspam-User: X-Stat-Signature: idg7iwqt1ehfue3mmt5er5wcfcgf4yx6 X-HE-Tag: 1791880847-415969 X-HE-Meta: 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 2ElgDqEE 6+rPT5ZToG3QfDLyDyw0UtAuZ3rDzUhc1KscUuwiMCgVczxZaJYUzUVsNMoA1UjSzIXri23skkNQV9FA2nZFId26LhA8bq5j2dx0Sy/5tcCB7T8VBm5Ov0I41epI/Rb8wlQ4VbUpEx8N2Prwb0X2749ez/kZ6o/ozBQNu/uNPZtVl/evxiu9fHQd2OFungpb4d0Fyn0T2PHcg4mQGCtCxVYZ3As10QtdwhjZBPhulPUfe87uFH52+fmA1kTMVGivneLiAeF9Atz+M50YWqHVMaBQbRXmcwKMq5uEVp0givfCY5qxTrsJHR7in9mDz2yRHX7nph/PzZuw+/MeaC4u/XNe9fw6mtCBGE+xGnE2z/7YXQVCrjzsyDo2ymlN7LWaX8aOt8KTvg5qAalA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Sep 28, 2026 at 05:33:13PM +0530, Ojaswin Mujoo wrote: > In iomap_writethrough_iter() we might encounter repeating folios across > multiple iterations. Repeating folios can occur if, example, > copy_folio_from_iter_atomic() does a short copy due to userspace pages > not faulted in. This is an issue because a previous loop might have > started writeback on them but not yet issued the IO. In the next > iteration trying to get the same folio with FGP_STABLE will result in a > deadlock. Since repeating folios will always be encountered back to > back, we can just use a simple cur != prev check to detect them. > > Use this to avoid waiting for writeback or starting writeback on folios > we have already processed. Note that in ->endio() we might end up > calling folio_end_writethrough() twice on the same folio which can cause > issues with folio_xor_flags_has_waiters(). For simplicity, just change > the folio_xor_flags_has_waiters() call to an idempotent variant. Is this an outdated comment? > > Reported-by: Pankaj Raghav Thanks for fixing it. I think I was able to trigger this in a VM with limited memory where the chances of short copy is probably higher. The fixes makes sense to me. I will run some tests this week and report back if there is an issue. -- Pankaj