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

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Message-Id: <437C30AA-2256-4F4F-9CC0-363A21DB1044@gmail.com>
Date:   Tue, 22 Jun 2021 11:16:03 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        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 Jun 22, 2021, at 11:07 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> 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.

I did send an RFC some time ago for “prepare” fault, but it the RFC
requires some more work.

https://lore.kernel.org/lkml/20210225072910.2811795-1-namit@vmware.com/