linux-kernel - Re: patch: improve generic_file_buffered

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Message-Id: <200709081725.06343.nickpiggin@yahoo.com.au>
Date:	Sat, 8 Sep 2007 17:25:06 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
Cc:	Bernd Schubert <bs@...eap.de>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	linux-kernel@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>, brian@...sterfs.com
Subject: Re: patch: improve generic_file_buffered_write() (2nd try 1/2)

On Saturday 08 September 2007 07:12, Goswin von Brederlow wrote:
> Nick Piggin <nickpiggin@...oo.com.au> writes:
> > On Saturday 08 September 2007 06:01, Goswin von Brederlow wrote:

> >>   b) a segment boundary
> >
> > This is done, as I said, because of the deadlock issue. While the issue
> > is more completely fixed in -mm, a special case for kernel memory (eg.
> > nfsd) is in the latest mainline kernels.
>
> Can you tell me where to get the fix from -mm? If it is completly
> fixed there then that could make our patch obsolete.

In the latest -mm series file, they start at
mm-revert-kernel_ds-buffered-write-optimisation.patch
...
and go to
ocfs2-convert-to-new-aops.patch


> >> What actually locks the page? Is it __grab_cache_page or
> >> a_ops->prepare_write?
> >
> > prepare_write must be given a locked page.
>
> Then that means __grab_cache_page does return a locked page because
> there is nothing between the two calls that would.

That's right.


> > No it would be included earlier. The "segment_eq" check should be
> > allowing kernel writes (nfsd) to write multiple segments. If you have a
> > patch which changes this significantly, then it would indicate the
> > existing logic has a problem (or you've got a userspace application doing
> > the writev, which should be fixed by the write_begin patches in -mm).
>
> I've got userspace application doing the writev. To be exact 14% of
> the commits were saved by combining multiple segments into a single
> prepare/write pair. Since the kernel segments don't fragment anymore
> in 2.6.23-rc5 those savings must come from user space stuff.
>
> From the stats posted earlier you can see that there is a substantial
> amount of calls with 6 segments all (alot) smaller than a page. Lots
> of calls our patch or the write_begin/end will save.

OK. The write_begin/write_end patchset is intrusive, no question. I'm not sure
what you're intending to do with it. They have been tested in -mm for quite a
while now, but just going with a simple patch that tries to copy more segments
might be OK for you if you're backporting. The deadlock is pretty uncommon.
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