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 09:39:08 -0700
From:	Jeremy Allison <jra@...ba.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	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, Mar 27, 2015 at 09:30:46AM -0700, Andrew Morton wrote:
> 
> But from an interface perspective the behaviour you're asking for is
> insane, frankly - if the kernel copied out 8k of data then pread2()
> should return 8k.  Otherwise there's no way for userspace to know that
> the 8k copy actually happened and we have just wasted a great pile of
> CPU doing a pointless memcpy.

Why would it do the copy in the first place if we asked (for example)
for 16k, but only 8k was available ? Just return EAGAIN and have
done with it.

> 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.

We can do that, but as Volker says this is a very hot code path.

> I take it from your comments that nobody has actually wired up pread2()
> into samba yet?  That's a bit disturbing, because if we later want to
> go and change something like this short-read behaviour, we're screwed -
> it's a non back-compat userspace-visible change.

It's been done as a test, so the code exists and has run (and improved
perforamance as I recall). Not much point commiting it without kernel
support :-).

> And a note on cosmetics: why are we using EAGAIN here rather than
> EWOULDBLOCK?  They have the same numerical value, but EWOULDBLOCK is a
> better name - EAGAIN says "run it again", but that won't work.

Sounds good to me !
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