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Mon, 11 Nov 2024 13:25:31 -0800 (PST) MIME-Version: 1.0 References: <20241110152906.1747545-1-axboe@kernel.dk> In-Reply-To: From: Yu Zhao Date: Mon, 11 Nov 2024 14:24:54 -0700 Message-ID: Subject: Re: [PATCHSET v2 0/15] Uncached buffered IO To: Matthew Wilcox Cc: Jens Axboe , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, hannes@cmpxchg.org, clm@meta.com, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 22A3518000D X-Stat-Signature: 54inq8a8w7z9bumi1x685djauw8rbo8b X-Rspam-User: X-HE-Tag: 1731360326-554174 X-HE-Meta: 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 itt1T83g 943zta8I6S8oj/cNl0qPPHuoVEjc/L7zROZWmo2G/VP4iv3+INk40ztYCQ2ouwjpXvUPSRYEyNoKGAy81kJuDsLbpC9qn7do9vOqe4itA/elrN6tV1pOLagEcFUw+N9SIDVQrtdTcRKmDJLkS/aj3kCUsRs7JT+KkXoYsNVpsHv6eq6dzaD3pWMd5vCFlIIB7KKKLWoZk+wlZgUUMjTsw4uBOF6qtM2xQ/87URIKbdfSGO6mgGuin3w6bkLI903zGMnw4/GiJghoRo2C3Q1mzGwbKJw== 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 Mon, Nov 11, 2024 at 10:25=E2=80=AFAM Matthew Wilcox wrote: > > On Sun, Nov 10, 2024 at 08:27:52AM -0700, Jens Axboe wrote: > > 5 years ago I posted patches adding support for RWF_UNCACHED, as a way > > to do buffered IO that isn't page cache persistent. The approach back > > then was to have private pages for IO, and then get rid of them once IO > > was done. But that then runs into all the issues that O_DIRECT has, in > > terms of synchronizing with the page cache. > > Today's a holiday, and I suspect you're going to do a v3 before I have > a chance to do a proper review of this version of the series. > > I think "uncached" isn't quite the right word. Perhaps 'RWF_STREAMING' > so that userspace is indicating that this is a streaming I/O and the > kernel gets to choose what to do with that information. > > Also, do we want to fail I/Os to filesystems which don't support > it? I suppose really sophisticated userspace might fall back to > madvise(DONTNEED), but isn't most userspace going to just clear the flag > and retry the I/O? > > Um. Now I've looked, we also have posix_fadvise(POSIX_FADV_NOREUSE), > which is currently a noop. Just to clarify that NOREUSE is NOT a noop since commit 17e8102 ("mm: support POSIX_FADV_NOREUSE"). And it had at least one user (we made the userpspace change after the kernel supported it): SVT-AV1 [1]; it was also added to FIO for testing purposes. [1] https://gitlab.com/AOMediaCodec/SVT-AV1 > But would we be better off honouring > POSIX_FADV_NOREUSE than introducing RWF_UNCACHED? I'll think about this > some more while I'm offline. But I guess the flag isn't honored the way UNCACHED works?