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Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user
 buffer pages"?
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <CAHk-=wicC9ZTNNH1E-oHebcT3+r4Q4Wf1tXBindXrCdotj20Gg@mail.gmail.com>
Date:   Tue, 22 Jun 2021 11:16:03 -0700
Cc:     Matthew Wilcox <willy@infradead.org>,
        David Howells <dhowells@redhat.com>,
        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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To:     Linus Torvalds <torvalds@linux-foundation.org>
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> On Jun 22, 2021, at 11:07 AM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:
>=20
> On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox <willy@infradead.org> =
wrote:
>>=20
>> 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.
>=20
> Yeah, we don't have any kind of async fault-in model.
>=20
> 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 =E2=80=9Cprepare=E2=80=9D fault, but =
it the RFC
requires some more work.

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