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

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Message-Id: <20150327093942.8ad38cd7.akpm@linux-foundation.org>
Date:	Fri, 27 Mar 2015 09:39:42 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeremy Allison <jra@...ba.org>,
	Christoph Hellwig <hch@...radead.org>,
	Milosz Tanski <milosz@...in.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>,
	linux-api@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-arch@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache
 only)

On Fri, 27 Mar 2015 09:30:46 -0700 Andrew Morton <akpm@...ux-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();
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