repair: don't grind CPUs with large extent lists

repair: don't grind CPUs with large extent lists

When repairing a large filesystem with fragemented files, xfs_repair
can grind to a halt burning multiple CPUs in blkmap_set_ext().
blkmap_set_ext() inserts extents into the blockmap for the inode
fork and it keeps them in order, even if the inserts are not done in
order.

The ordered insert is highly inefficient - it starts at the first
extent, and simple walks the array to find the insertion point. i.e.
it is an O(n) operation. When we have a fragemented file with a
large number of extents, the cost of the entire mapping operation
is rather costly.

The thing is, we are doing the insertion from an *ordered btree
scan* which is inserting the extents in ascending offset order.
IOWs, we are always inserting the extent at the end of the array
after searching the entire array. i.e. the mapping operation cost is
O(N^2).

Fix this simply by reversing the order of the insert slot search.
Start at the end of the blockmap array when we do almost all
insertions, which brings the overhead of each insertion down to O(1)
complexity. This, in turn, results in the overall map building
operation being reduced to an O(N) operation, and so performance
degrades linearly with increasing extent counts rather than
exponentially.

While there, I noticed that the growing of the blkmap array was only
done 4 extents at a time. When we are dealing with files that may
have hundreds of thousands of extents, growing th map only 4 extents
at a time requires excessive amounts of reallocation. Reduce the
reallocation rate by increasing the grow increment according to how
large the array currently is.

The result is that the test filesystem (27TB, 30M inodes, at ENOSPC)
takes 5m10s to *fully repair* on my test system, rather that getting
15 (of 60) AGs into phase three and sitting there burning 3-4 CPUs
making no progress for over half an hour.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>