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Date:   Tue, 22 Jun 2021 11:32:00 -0700
From:   Linus Torvalds <>
To:     David Howells <>
Cc:     Matthew Wilcox <>,
        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 Tue, Jun 22, 2021 at 11:23 AM David Howells <> wrote:
> Probably the most obvious way would be to set a flag in task_struct saying
> what you're doing and have the point that would otherwise wait for the page to
> become unlocked skip to the fault fixup code if the page is locked after
> ->readahead() has been invoked and the flag is set, then use get_user() in
> iov_iter_fault_in_readable().

Yeah, the existing user access exception handling code _almost_
handles it, except for one thing: you'd need to have some way to
distinguish between "prefetch successful" and "fault failed".

And yeah, I guess it could be a flag in task_struct, but at that point
my gag reflex starts acting up.


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