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

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Message-Id: <20150327013516.8c6788be.akpm@linux-foundation.org>
Date:	Fri, 27 Mar 2015 01:35:16 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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 Fri, 27 Mar 2015 01:18:22 -0700 Christoph Hellwig <hch@...radead.org> wrote:

> On Thu, Mar 26, 2015 at 08:28:24PM -0700, Andrew Morton wrote:
> > I still don't understand why pwritev() exists.  We discussed this last
> > time but it seems nothing has changed.  I'm not seeing here an adequate
> > description of why it exists nor a justification for its addition.
> 
> pwritev2?  I have patches to support per-I/O O_DSYNC with it, lots of
> folks including Samba and SCSI targets want this because their protocols
> support it.  The patches were posted with earlier versions of Miklos
> series.
> 
> It's cleaner to add the two system calls in go when we plan using them
> anyway and have symmetric infrastructure, and I did not hear any
> disagreement with that on LSF.  Did you skip this session?

Put it in the changelogs.  All of it.  A conference discussion
is no use to people who weren't there.

> > And (again) we've discussed this before, but the patchset gets resent
> > as if nothing had happened.
> 
> We had long discussiosn about it both here and at LSF.  We had everyone
> agree and nod there, and only your repeated argument here, so maybe it's
> not Miklos who is disonnected but you?

I don't find conferences to be a good place to conduct code and design
review.

> Also that whole fincore argument is rather hypothetic - it's only been
> pushed in to ugly to live multiplexers that also expose things like
> pfns,  while with preadv2 we have a trivial and easy to use API read to
> merge, and various consumerms just waiting for it.

fincore() doesn't have to be ugly.  Please address the design issues I
raised.  How is pread2() useful to the class of applications which
cannot proceed until all data is available?


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