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SubjectRe: [patch 04/27] fs: introduce mnt_clone_write
On Sat, Apr 25, 2009 at 11:20:24AM +1000, npiggin@suse.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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