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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 22 Jun 2021 11:32:00 -0700
Message-ID: <CAHk-=wgA4_TkMqOw9GwW7aNe3jBU_yBKZkNWTicz=BKap_=siw@mail.gmail.com>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?
To:     David Howells <dhowells@redhat.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>, "Ted Ts'o" <tytso@mit.edu>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jun 22, 2021 at 11:23 AM David Howells <dhowells@redhat.com> 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.

               Linus