linux-ext4 - Re: Do we need to unrevert "fs: do not prefault sys

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Date:   Tue, 22 Jun 2021 18:55:33 +0100
From:   David Howells <dhowells@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     dhowells@...hat.com, Matthew Wilcox <willy@...radead.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"?

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.

David