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

From: Andrew Morton <akpm@linux-foundation.org>
To: Jeremy Allison <jra@samba.org>,
	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 09:39:42 -0700	[thread overview]
Message-ID: <20150327093942.8ad38cd7.akpm@linux-foundation.org> (raw)
In-Reply-To: <20150327093046.53c2769a.akpm@linux-foundation.org>

On Fri, 27 Mar 2015 09:30:46 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> I expect that this situation (first part in cache, latter part not in
> cache) is rare - for reasonably small requests the common cases will be
> "all cached" and "nothing cached".  So perhaps the best approach here
> is for samba to add special handling for the short read, to work out
> the reason for its occurrence.
> 
> Alternatively we could add another flag to pread2() to select this
> "throw away my data and return -EAGAIN" behaviour.  Presumably
> implemented with an i_size check, but it's gonna be racy.

Here's a better way:

	nr_read = pread2(buf, len);
	if (nr_read < len)
		nr_read += pread(buf + nr_read, len - nr_read);
	if (nr_read < len)
		we_hit_eof();