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

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Message-Id: <E1YbQoP-008C4F-TU@intern.SerNet.DE>
Date:	Fri, 27 Mar 2015 10:44:09 +0100
From:	Volker Lendecke <Volker.Lendecke@...Net.DE>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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>, 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, Mar 27, 2015 at 02:01:59AM -0700, Andrew Morton wrote:
> 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.

Sorry, but I don't have a good picture how we are supposed
to use that. I'm fine with two syscalls, but I need a way to
tell the kernel to either block or not. Or do you want Samba
to do repeated pread calls for ever shorter blocks? Right
now I don't see a way to tell pread to either give me a
short result or really block. To me that's the core of
preadv2. I'm perfectly find for a syscall to give me a short
read instead of a global EWOULDBLOCK. I need a way to tell
the kernel which behaviour I want.

Volker

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