Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?

From: Matthew Wilcox <willy@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: 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>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?
Date: Tue, 22 Jun 2021 19:23:10 +0100	[thread overview]
Message-ID: <YNIqjhsvEms6+vk9@casper.infradead.org> (raw)
In-Reply-To: <CAHk-=wicC9ZTNNH1E-oHebcT3+r4Q4Wf1tXBindXrCdotj20Gg@mail.gmail.com>

On Tue, Jun 22, 2021 at 11:07:56AM -0700, Linus Torvalds wrote:
> On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > 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.
> 
> Yeah, we don't have any kind of async fault-in model.
> 
> 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.

It wouldn't be _that_ bad necessarily.  filemap_fault:

        page = find_get_page(mapping, offset);
...
        } else if (!page) {
                fpin = do_sync_mmap_readahead(vmf);

... and we could return at that point if the flag was set.  There'd be
some more details to fill in (if there's a !uptodate page in the page
cache, don't wait for it), but it might not be too bad.