Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)

From: Jeremy Allison <jra@samba.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	Milosz Tanski <milosz@adfin.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-aio@kvack.org, Mel Gorman <mgorman@suse.de>,
	Volker Lendecke <Volker.Lendecke@sernet.de>,
	Tejun Heo <tj@kernel.org>, Jeff Moyer <jmoyer@redhat.com>,
	"Theodore Ts'o" <tytso@mit.edu>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-api@vger.kernel.org,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	linux-arch@vger.kernel.org, Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)
Date: Fri, 27 Mar 2015 08:58:54 -0700	[thread overview]
Message-ID: <20150327155854.GA5548@samba2> (raw)
In-Reply-To: <20150327020159.eadd0ce1.akpm@linux-foundation.org>

On Fri, Mar 27, 2015 at 02:01:59AM -0700, Andrew Morton wrote:
> On Fri, 27 Mar 2015 01:48:33 -0700 Christoph Hellwig <hch@infradead.org> wrote:
> 
> > On Fri, Mar 27, 2015 at 01:35:16AM -0700, Andrew Morton wrote:
> > > fincore() doesn't have to be ugly.  Please address the design issues I
> > > raised.  How is pread2() useful to the class of applications which
> > > cannot proceed until all data is available?
> > 
> > It actually makes them work correctly?  preadv2( ..., DONTWAIT) will
> > return -EGAIN, which causes them to bounce to the threadpool where
> > they call preadv(...).
> 
> (I assume you mean RWF_NONBLOCK)
> 
> That isn't how pread2() works.  If the leading one or more pages are
> uptodate, pread2() will return a partial read.  Now what?  Either the
> application reads the same data a second time via the worker thread
> (dumb, but it will usually be a rare case)

The problem with the above is that we can't tell the difference
between pread2() returning a short read because the pages are not
in cache, or because someone truncated the file. So we need some
way to differentiate this.

My preference from userspace would be for pread2() to return
EAGAIN if *all* the data requested is not available (where
'all' can be less than the size requested if the file has
been truncated in the meantime).

So:

ret = pread2(fd, buf, size_wanted, RWF_NONBLOCK)

if (ret == -1) {
	if (errno == EAGAIN) {
		goto threadpool...
	}
	.. real error..
}

if (ret == size_wanted) {
	.. normal read, file not truncated...
}

if (ret < size_wanted) {
	.. file was truncated..
}

The thing I want to avoid is the case where
ret < size_wanted means only part of the file
is in cache.