[PATCH v3 4/9] xfsrestore: mmap dirent names for faster lookups

[Christoph Hellwig]

On Tue, Nov 16, 2010 at 09:05:06AM -0600, wkendall at sgi.com wrote:
> Pathname resolution in xfsrestore is about 4x faster if the file
> containing dirent names ("namreg") is memory mapped.  If xfsrestore is
> unable to map the file (e.g., due to virtual memory constraints)
> fallback to the existing seek-and-read approach.
> 
> The file is mapped after all directory entries have been written to
> the "namreg" file. If the caller tries to add additional entries after
> the file has been mapped, it will be unmapped and restore will resort
> back to seek-and-read lookups.
> 
> Signed-off-by: Bill Kendall <wkendall at sgi.com>
> 
> Reviewed-by: Alex Elder <aelder at sgi.com>

Generally looks good to me, but I really hate how namreg_map/unmap
are hidden under namreg_add/flush.  As a start instead of adding the
done_adding argument we can easily move the explicit map to the one
caller wanting it, similarly namreg_unmap could be moved to namreg_add,
that is manage to mapping/unmapping explicitly.

In fact I'm not sure what the point of the unmap/map cycles is.  At
least in Linux concurrent buffer writes and mmap reads are coherent.