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Date:   Tue, 22 Jun 2021 19:36:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 11:28:30AM -0700, Linus Torvalds wrote:
> 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.

Oh, I was assuming that it'd be a function call like
get_user_pages_fast(), not an instruction that was specially marked to
be jumped over.  Gag reflex diminishing now?

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