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[2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-427ea5a0f19sm19610031f8f.9.2025.10.21.02.37.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Oct 2025 02:37:58 -0700 (PDT) Message-ID: <750cfcac-e048-4fee-bba9-6e84edb7bbe0@redhat.com> Date: Tue, 21 Oct 2025 11:37:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO To: Jan Kara , Christoph Hellwig Cc: Matthew Wilcox , Qu Wenruo , linux-btrfs@vger.kernel.org, djwong@kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, martin.petersen@oracle.com, jack@suse.com References: <1ee861df6fbd8bf45ab42154f429a31819294352.1760951886.git.wqu@suse.com> <56o3re2wspflt32t6mrfg66dec4hneuixheroax2lmo2ilcgay@zehhm5yaupav> <5bd1d360-bee0-4fa2-80c8-476519e98b00@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: x3cfkffbjtdw6scqbyop7ra5z6sxg4gt X-Rspamd-Queue-Id: 3E90A160008 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1761039484-335849 X-HE-Meta: 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 JmsPwxTH 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 21.10.25 11:22, Jan Kara wrote: > On Tue 21-10-25 00:49:49, Christoph Hellwig wrote: >> On Mon, Oct 20, 2025 at 09:00:50PM +0200, David Hildenbrand wrote: >>> Just FYI, because it might be interesting in this context. >>> >>> For anonymous memory we have this working by only writing the folio out if >>> it is completely unmapped and there are no unexpected folio references/pins >>> (see pageout()), and only allowing to write to such a folio ("reuse") if >>> SWP_STABLE_WRITES is not set (see do_swap_page()). >>> >>> So once we start writeback the folio has no writable page table mappings >>> (unmapped) and no GUP pins. Consequently, when trying to write to it we can >>> just fallback to creating a page copy without causing trouble with GUP pins. >> >> Yeah. But anonymous is the easy case, the pain is direct I/O to file >> mappings. Mapping the right answer is to just fail pinning them and fall >> back to (dontcache) buffered I/O. > > I agree file mappings are more painful but we can also have interesting > cases with anon pages: > > P - anon page > > Thread 1 Thread 2 > setup DIO read to P setup DIO write from P Ah, I was talking about the interaction between GUP and having BLK_FEAT_STABLE_WRITES set on the swap backend. I guess what you mean here is: GUP from/to anon pages to/from a device that has BLK_FEAT_STABLE_WRITES? So while we are writing to the device using the anon page as a source, the anon page will get modified. I did not expect that to trigger checksum failures, but I can see the problem now. -- Cheers David / dhildenb