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

From: Volker Lendecke <Volker.Lendecke@SerNet.DE>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Milosz Tanski <milosz@adfin.com>,
	linux-kernel@vger.kernel.org,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
	Mel Gorman <mgorman@suse.de>, Tejun Heo <tj@kernel.org>,
	Jeff Moyer <jmoyer@redhat.com>, "Theodore Ts'o" <tytso@mit.edu>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-api@vger.kernel.org,
	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)
Date: Fri, 27 Mar 2015 06:41:25 +0100	[thread overview]
Message-ID: <E1YbN1J-0084qO-3s@intern.SerNet.DE> (raw)
In-Reply-To: <20150326202824.65d03787.akpm@linux-foundation.org>

On Thu, Mar 26, 2015 at 08:28:24PM -0700, Andrew Morton wrote:
> A thing which bugs me about pread2() is that it is specifically
> tailored to applications which are able to use a partial read result. 
> ie, by sending it over the network.

Can you explain what you mean by this? Samba gets a pread
request from a client for some bytes. The client will be
confused when we send less than requested although the file
is long enough to satisfy all.

> And of course fincore could be used by Samba etc to avoid blocking on
> reads.  It wouldn't perform quite as well as pread2(), but I bet it's
> good enough.
> 
> Bottom line: with pread2() there's still a need for fincore(), but with
> fincore() there probably isn't a need for pread2().

fincore would be a second syscall per pread, and it is not
atomic. I've had discussions with MIPS based vendors who
are worried about every single syscall. This is the #1
hottest code path in Samba.

> And I'm doubtful about claims that it absolutely has to be non-blocking
> 100% of the time.  I bet that 99.99% is good enough.  A fincore()
> option to run mark_page_accessed() against present pages would help
> with the race-with-reclaim situation.

If you can make sure that after an fincore the pages remain
in memory for x milliseconds the atomicity concern might go
away.

Volker

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