linux-kernel - Re: [patch 04/27] fs: introduce mnt_clone

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Message-ID: <20090425033503.GV8633@ZenIV.linux.org.uk>
Date:	Sat, 25 Apr 2009 04:35:03 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	npiggin@...e.de
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [patch 04/27] fs: introduce mnt_clone_write

On Sat, Apr 25, 2009 at 11:20:24AM +1000, npiggin@...e.de wrote:
> This patch speeds up lmbench lat_mmap test by about another 2% after the
> first patch.
> 
> Before:
>  avg = 462.286
>  std = 5.46106
> 
> After:
>  avg = 453.12
>  std = 9.58257
> 
> (50 runs of each, stddev gives a reasonable confidence)
> 
> It does this by introducing mnt_clone_write, which avoids some heavyweight
> operations of mnt_want_write if called on a vfsmount which we know already
> has a write count; and mnt_want_write_file, which can call mnt_clone_write
> if the file is open for write.
> 
> After these two patches, mnt_want_write and mnt_drop_write go from 7% on
> the profile down to 1.3% (including mnt_clone_write).

NAK in this form; nested mnt_want_write() *CAN* fail (note the check for
superblock itself being r/o).  Make you mnt_clone_write() returning int
and doing that superblock check, and I'm OK with it.
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