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

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Date:   Tue, 22 Jun 2021 19:23:10 +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 11:07:56AM -0700, Linus Torvalds wrote:
> On Tue, Jun 22, 2021 at 11:05 AM Matthew Wilcox <willy@...radead.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.