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Date: Fri, 27 Mar 2015 02:01:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Milosz Tanski <milosz@adfin.com>, linux-kernel@vger.kernel.org,
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        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)
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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) 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.