linux-ext4 - Re: Do we need to unrevert "fs: do not prefault sys

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Date:   Tue, 22 Jun 2021 11:28:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     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 Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> It wouldn't be _that_ bad necessarily.  filemap_fault:

It's not actually the mm code that is the biggest problem. We
obviously already have readahead support.

It's the *fault* side.

In particular, since the fault would return without actually filling
in the page table entry (because the page isn't ready yet, and you
cannot expose it to other threads!), you also have to jump over the
instruction that caused this all.

Doable? Oh, absolutely. But you basically need a new inline asm thing
for each architecture, so that the architecture knows how to skip the
instruction that caused the page fault, and the new exception table
entry type to say 'this is that magic VM prefetch instruction".

So filemap_fault() is the least of the problems.

Honestly, it doesn't look _complicated_, but it does look like a fair
number of small details..

               Linus