linux-kernel - Re: Do we need to unrevert "fs: do not prefault sys

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Date:   Tue, 22 Jun 2021 19:04:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, Ted Ts'o <tytso@....edu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?

On Tue, Jun 22, 2021 at 06:55:33PM +0100, David Howells wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> 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.

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.