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[24.6.216.183]) by smtp.gmail.com with ESMTPSA id c3sm42759pfl.42.2021.06.22.11.16.05 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Jun 2021 11:16:06 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.100.0.2.22\)) Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? From: Nadav Amit In-Reply-To: Date: Tue, 22 Jun 2021 11:16:03 -0700 Cc: Matthew Wilcox , David Howells , Al Viro , Ted Ts'o , Dave Hansen , Andrew Morton , Linux-MM , Ext4 Developers List , linux-fsdevel , Linux Kernel Mailing List Content-Transfer-Encoding: quoted-printable Message-Id: <437C30AA-2256-4F4F-9CC0-363A21DB1044@gmail.com> References: <3221175.1624375240@warthog.procyon.org.uk> <3231150.1624384533@warthog.procyon.org.uk> To: Linus Torvalds X-Mailer: Apple Mail (2.3654.100.0.2.22) X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 610CF60019D1 X-Stat-Signature: wodspduicwhj3xxnuy1qkghm4oh451i8 Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=NFhodhGH; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf25.hostedemail.com: domain of nadavamit@gmail.com designates 209.85.215.170 as permitted sender) smtp.mailfrom=nadavamit@gmail.com X-HE-Tag: 1624385768-224284 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: > On Jun 22, 2021, at 11:07 AM, Linus Torvalds = wrote: >=20 > On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox = wrote: >>=20 >> Huh? Last I checked, the fault_in_readable actually read a byte from >> the page. It has to wait for the read to complete before that can >> happen. >=20 > Yeah, we don't have any kind of async fault-in model. >=20 > I'm not sure how that would even look. I don't think it would > necessarily be *impossible* (special marker in the exception table to > let the fault code know that this is a "prepare" fault), but it would > be pretty challenging. I did send an RFC some time ago for =E2=80=9Cprepare=E2=80=9D fault, but = it the RFC requires some more work. https://lore.kernel.org/lkml/20210225072910.2811795-1-namit@vmware.com/