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

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Date:	Fri, 27 Mar 2015 02:01:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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 01:48:33 -0700 Christoph Hellwig <hch@...radead.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) or it reads the remainder of
the data in the worker thread and splices the data back together. 
Which, as I said, will often result in a second load of the initial
read result into CPU cache.
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