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Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <3221175.1624375240@warthog.procyon.org.uk> To: Linus Torvalds Cc: dhowells@redhat.com, Matthew Wilcox , Al Viro , "Ted Ts'o" , Dave Hansen , Andrew Morton , Linux-MM , Ext4 Developers List , linux-fsdevel , Linux Kernel Mailing List Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <3231149.1624384533.1@warthog.procyon.org.uk> Date: Tue, 22 Jun 2021 18:55:33 +0100 Message-ID: <3231150.1624384533@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=LNqYMMEw; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf30.hostedemail.com: domain of dhowells@redhat.com has no SPF policy when checking 216.205.24.124) smtp.mailfrom=dhowells@redhat.com X-Rspamd-Server: rspam02 X-Stat-Signature: 51jsrdpgc7bygbdgz1cbwm44qg5ba7ym X-Rspamd-Queue-Id: D2BD0E000262 X-HE-Tag: 1624384541-115757 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: Linus Torvalds wrote: > End result: doing the fault_in_readable "unnecessarily" at the > beginning is likely the better optimization. It's basically free when > it's not necessary, and it avoids an extra fault (and extra > lock/unlock and retry) when it does end up faulting pages in. It may also cause the read in to happen in the background whilst write_begin is being done. David