We process the buf_log_item bitmap one set bit at a time with
xfs_next_bit() so we can detect if a region crosses a memcpy
discontinuity in the buffer data address. This has massive overhead
on large buffers (e.g. 64k directory blocks) because we do a lot of
unnecessary checks and xfs_buf_offset() calls.

For example, 16-way concurrent create workload on debug kernel
running CPU bound has this at the top of the profile at ~120k
create/s on 64kb directory block size:

  20.66%  [kernel]  [k] xfs_dir3_leaf_check_int
   7.10%  [kernel]  [k] memcpy
   6.22%  [kernel]  [k] xfs_next_bit
   3.55%  [kernel]  [k] xfs_buf_offset
   3.53%  [kernel]  [k] xfs_buf_item_format
   3.34%  [kernel]  [k] __pv_queued_spin_lock_slowpath
   3.04%  [kernel]  [k] do_raw_spin_lock
   2.84%  [kernel]  [k] xfs_buf_item_size_segment.isra.0
   2.31%  [kernel]  [k] __raw_callee_save___pv_queued_spin_unlock
   1.36%  [kernel]  [k] xfs_log_commit_cil

(debug checks hurt large blocks)

The only buffers with discontinuities in the data address are
unmapped buffers, and they are only used for inode cluster buffers
and only for logging unlinked pointers. IOWs, it is -rare- that we
even need to detect a discontinuity in the buffer item formatting
code.

Optimise all this by using xfs_contig_bits() to find the size of
the contiguous regions, then test for a discontiunity inside it. If
we find one, do the slow "bit at a time" method we do now. If we
don't, then just copy the entire contiguous range in one go.

Profile now looks like:

  25.26%  [kernel]  [k] xfs_dir3_leaf_check_int
   9.25%  [kernel]  [k] memcpy
   5.01%  [kernel]  [k] __pv_queued_spin_lock_slowpath
   2.84%  [kernel]  [k] do_raw_spin_lock
   2.22%  [kernel]  [k] __raw_callee_save___pv_queued_spin_unlock
   1.88%  [kernel]  [k] xfs_buf_find
   1.53%  [kernel]  [k] memmove
   1.47%  [kernel]  [k] xfs_log_commit_cil
....
   0.34%  [kernel]  [k] xfs_buf_item_format
....
   0.21%  [kernel]  [k] xfs_buf_offset
....
   0.16%  [kernel]  [k] xfs_contig_bits
....
   0.13%  [kernel]  [k] xfs_buf_item_size_segment.isra.0

So the bit scanning over for the dirty region tracking for the
buffer log items is basically gone. Debug overhead hurts even more
now...

Perf comparison

		dir block	 creates		unlink
		size (kb)	time	rate		time

Original	 4		4m08s	220k		 5m13s
Original	64		7m21s	115k		13m25s
Patched		 4		3m59s	230k		 5m03s
Patched		64		6m23s	143k		12m33s

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>