linux-kernel - Re: patch: improve generic_file_buffered

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Message-ID: <87hcm6chtb.fsf@informatik.uni-tuebingen.de>
Date:	Fri, 07 Sep 2007 23:00:00 +0200
From:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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,
	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
Subject: Re: patch: improve generic_file_buffered_write() (2nd try 1/2)

Nick Piggin <nickpiggin@...oo.com.au> writes:

> Anyway, there are fixes for this deadlock in Andrew's -mm tree, but
> also a workaround for the NFSD problem in git commit 29dbb3fc. Did
> you try a later kernel to see if it is fixed there?

I had a chance to look up that commit (git clone took a while so sorry
for writing 2 mails). It is present in 2.6.23-rc5 so I already noticed
it when merging our patch in 2.6.23-rc5.

Upon closer reading of the patch though I see that it will indeed
prevent writes by the nfsd to be split smaller than PAGE_SIZE and it
will cause filemap_copy_from_user[_iovec] to be called with a source
spanning multiple pages.

So the commit 29dbb3fc should have a simmilar, slightly better even,
gain for the nfsd and other kernel space segments. But it will not
improve writes from user space, where ~14% of the commits were saved
during a days work for me.

Now I have a question about fault_in_pages_readable(). Can I call that
for multiple pages and then call __grab_cache_page() without risking
one of the pages from getting lost again and causing a deadlock?

MfG
        Goswin
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