btrfs: search for larger extent maps inside btrfs_do_readpage()

btrfs: search for larger extent maps inside btrfs_do_readpage()

[CORNER CASE]
If we have the following file extents layout, btrfs_get_extent() can
return a smaller hole during read, and cause unnecessary extra tree
searches:

	item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
		generation 9 type 1 (regular)
		extent data disk byte 13631488 nr 4096
		extent data offset 0 nr 4096 ram 4096
		extent compression 0 (none)

	item 7 key (257 EXTENT_DATA 32768) itemoff 15757 itemsize 53
		generation 9 type 1 (regular)
		extent data disk byte 13635584 nr 4096
		extent data offset 0 nr 4096 ram 4096
		extent compression 0 (none)

In above case, range [0, 4K) and [32K, 36K) are regular extents, and
there is a hole in range [4K, 32K), and the fs has "no-holes" feature,
meaning the hole will not have a file extent item.

[INEFFICIENCY]
Assume the system has 4K page size, and we're doing readahead for range
[4K, 32K), no large folio yet.

 btrfs_readahead() for range [4K, 32K)
 |- btrfs_do_readpage() for folio 4K
 |  |- get_extent_map() for range [4K, 8K)
 |     |- btrfs_get_extent() for range [4K, 8K)
 |        We hit item 6, then for the next item 7.
 |        At this stage we know range [4K, 32K) is a hole.
 |        But our search range is only [4K, 8K), not reaching 32K, thus
 |        we go into not_found: tag, returning a hole em for [4K, 8K).
 |
 |- btrfs_do_readpage() for folio 8K
 |  |- get_extent_map() for range [8K, 12K)
 |     |- btrfs_get_extent() for range [8K, 12K)
 |        We hit the same item 6, and then item 7.
 |        But still we goto not_found tag, inserting a new hole em,
 |        which will be merged with previous one.
 |
 | [ Repeat the same btrfs_get_extent() calls until the end ]

So we're calling btrfs_get_extent() again and again, just for a
different part of the same hole range [4K, 32K).

[ENHANCEMENT]
Make btrfs_do_readpage() to search for a larger extent map if readahead
is involved.

For btrfs_readahead() we have bio_ctrl::ractl set, and lock extents for
the whole readahead range.

If we find bio_ctrl::ractl is set, we can use that end range as extent
map search end, this allows btrfs_get_extent() to return a much larger
hole, thus reduce the need to call btrfs_get_extent() again and again.

 btrfs_readahead() for range [4K, 32K)
 |- btrfs_do_readpage() for folio 4K
 |  |- get_extent_map() for range [4K, 32K)
 |     |- btrfs_get_extent() for range [4K, 32K)
 |        We hit item 6, then for the next item 7.
 |        At this stage we know range [4K, 32K) is a hole.
 |        So the hole em for range [4K, 32K) is returned.
 |
 |- btrfs_do_readpage() for folio 8K
 |  |- get_extent_map() for range [8K, 32K)
 |     The cached hole em range [4K, 32K) covers the range,
 |     and reuse that em.
 |
 | [ Repeat the same btrfs_get_extent() calls until the end ]

Now we only call btrfs_get_extent() once for the whole range [4K, 32K),
other than the old 8 times.

Such change will reduce the overhead of reading large holes a little.
For current experimental build (with larger folios) on aarch64, there
will be a tiny but consistent ~1% improvement reading a large hole file:

 Reading a 1GiB sparse file (all hole) using xfs_io, with 64K block
 size, the result is the time needed to read the whole file, reported
 from xfs_io.

 32 runs, experimental build (with large folios).

 64K page size, 4K fs block size.

 - Avg before: 0.20823 s
 - Avg after:  0.20635 s
 - Diff:   -0.9%

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>