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Date:   Tue, 22 Jun 2021 11:16:03 -0700
From:   Nadav Amit <>
To:     Linus Torvalds <>
Cc:     Matthew Wilcox <>,
        David Howells <>,
        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"?

> On Jun 22, 2021, at 11:07 AM, Linus Torvalds <> wrote:
> On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox <> 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.

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