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[2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-427f00ce3aesm19874869f8f.48.2025.10.21.02.43.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Oct 2025 02:43:06 -0700 (PDT) Message-ID: Date: Tue, 21 Oct 2025 11:43:05 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Cc: Christoph Hellwig , 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> <32a9b501-742d-4954-9207-bb7d0c08fccb@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 21.10.25 11:33, Jan Kara wrote: > On Tue 21-10-25 09:57:08, David Hildenbrand wrote: >> On 21.10.25 09:49, 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. >> >> Right, I think the rules could likely be >> >> a) Don't start writeback to such devices if there may be GUP pins (o >> writeble PTEs) >> >> b) Don't allow FOLL_WRITE GUP pins if there is writeback to such a device >> >> Regarding b), I would have thought that GUP would find the PTE to not be >> writable and consequently trigger a page fault first to make it writable? >> And I'd have thought that we cannot make such a PTE writable while there is >> writeback to such a device going on (otherwise the CPU could just cause >> trouble). > > See some of the cases in my reply to Christoph. It is also stuff like: > > c) Don't allow FOLL_WRITE GUP pins or writeable mapping if there are *any* > pins to the page. > > And we'd have to write-protect the page in the page tables at the moment we > obtain the FOLL_WRITE GUP pin to make sure the pin owner is the only thread > able to modify that page contents while the DIO is running. Oh that's nasty, but yeah I understood the problem now, thanks. -- Cheers David / dhildenb