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 10:19:12 -0700
From:	Jeremy Allison <jra@...ba.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Jeremy Allison <jra@...ba.org>,
	Andrew Morton <akpm@...ux-foundation.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 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.
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