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 19:09:43 -0400
From:	Milosz Tanski <milosz@...in.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jeremy Allison <jra@...ba.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 3:36 AM, Christoph Hellwig <hch@...radead.org> 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?
>
>> 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.

I'm okay with an old or nothing flag. Although I think that would much
more useful with RWF_NONWAIT with pwritev, in applications that don't
want to block while logging (but it's okay to drop low level log
messages). That's a whole different use case in my mind.

-- 
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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