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

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Date:	Mon, 30 Mar 2015 18:51:50 -0400
From:	Milosz Tanski <milosz@...in.com>
To:	Jeremy Allison <jra@...ba.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-aio@...ck.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 <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 Mon, Mar 30, 2015 at 1:19 PM, Jeremy Allison <jra@...ba.org> wrote:
> On Mon, Mar 30, 2015 at 12:36:04AM -0700, Christoph Hellwig wrote:
>> On Fri, Mar 27, 2015 at 08:58:54AM -0700, Jeremy Allison wrote:
>> > The problem with the above is that we can't tell the difference
>> > between pread2() returning a short read because the pages are not
>> > in cache, or because someone truncated the file. So we need some
>> > way to differentiate this.
>>
>> Is a race vs truncate really that time critical that you can't
>> wait for the thread pool to do the second read to notice it?
>
> Probably not, as this is the fallback path anyway.
>
>> > My preference from userspace would be for pread2() to return
>> > EAGAIN if *all* the data requested is not available (where
>> > 'all' can be less than the size requested if the file has
>> > been truncated in the meantime).
>>
>> That is easily implementable, but I can see that for example web apps
>> would be happy to get as much as possible.  So if Samba can be ok
>> with short reads and only detecting the truncated case in the slow
>> path that would make life simpler.  Otherwise we might indeed need two
>> flags.
>
> Simpler is better. I can live with the partial read+fallback.
>
> Jeremy.

The partial behavior is very useful for protocols like HTTP1, since
the client can start processing the response if we send it down the
wire while we process other connections. It becomes even more useful
over HTTP2 which provides it's own framing where we can send a partial
response frame and move onto other requests in this connection or
other connections.

-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@...in.com
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