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

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Message-ID: <CAHk-=wicC9ZTNNH1E-oHebcT3+r4Q4Wf1tXBindXrCdotj20Gg@mail.gmail.com>
Date:   Tue, 22 Jun 2021 11:07:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     David Howells <dhowells@...hat.com>,
        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 11:05 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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.

Yeah, we don't have any kind of async fault-in model.

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.

            Linus