linux-kernel - Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only)

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Message-ID: <20140919104204.3b0bb762@lwn.net>
Date:	Fri, 19 Sep 2014 10:42:04 -0400
From:	Jonathan Corbet <corbet@....net>
To:	Milosz Tanski <milosz@...in.com>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.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>
Subject: Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only)

On Wed, 17 Sep 2014 22:20:45 +0000
Milosz Tanski <milosz@...in.com> wrote:

> This patcheset introduces an ability to perform a non-blocking read from
> regular files in buffered IO mode. This works by only for those filesystems
> that have data in the page cache.
> 
> It does this by introducing new syscalls new syscalls readv2/writev2 and
> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg
> syscalls that accept an extra flag argument (O_NONBLOCK).

So I'm trying to understand the reasoning behind this approach so I can
explain it to others.  When you decided to add these syscalls, you
ruled out some other approaches that have been out there for a while.
I assume that, before these syscalls can be merged, people will want to
understand why you did that.  So I'll ask the dumb questions:

 - Non-blocking I/O has long been supported with a well-understood set
   of operations - O_NONBLOCK and fcntl().  Why do we need a different
   mechanism here - one that's only understood in the context of
   buffered file I/O?  I assume you didn't want to implement support
   for poll() and all that, but is that a good enough reason to add a
   new Linux-specific non-blocking I/O technique?

 - Patches adding fincore() have been around since at least 2010; see,
   for example, https://lwn.net/Articles/371538/ or
   https://lwn.net/Articles/604640/.  It seems this could be used in
   favor of four new read() syscalls; is there a reason it's not
   suitable for your use case?

 - Patches adding buffered support for AIO have been around since at
   least 2003 - https://lwn.net/Articles/24422/, for example.  I guess
   I don't really have to ask why you don't want to take that
   approach! :)  

Apologies for my ignorance here; that's what I get for hanging around
with the MM folks at LSFMM, I guess.  Anyway, I suspect I'm not the
only one who would appreciate any background you could give here.

Thanks,

jon
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