btrfs: make lseek and fiemap much more efficient

From: Filipe Manana <fdmanana@suse.com>

We often get reports of fiemap and hole/data seeking (lseek) being too slow
on btrfs, or even unusable in some cases due to being extremely slow.

Some recent reports for fiemap:

    https://lore.kernel.org/linux-btrfs/21dd32c6-f1f9-f44a-46...
    https://lore.kernel.org/linux-btrfs/Ysace25wh5BbLd5f@atma...

For lseek (LSF/MM from 2017):

   https://lwn.net/Articles/718805/

Basically both are slow due to very high algorithmic complexity which
scales badly with the number of extents in a file and the heigth of
subvolume and extent b+trees.

Using Pavel's test case (first Link tag for fiemap), which uses files with
many 4K extents and holes before and after each extent (kind of a worst
case scenario), the speedup is of several orders of magnitude (for the 1G
file, from ~225 seconds down to ~0.1 seconds).

Finally the new algorithm for fiemap also ends up solving a bug with the
current algorithm. This happens because we are currently relying on extent
maps to report extents, which can be merged, and this may cause us to
report 2 different extents as a single one that is not shared but one of
them is shared (or the other way around). More details on this on patches
9/10 and 10/10.

Patches 1/10 and 2/10 are for lseek, introducing some code that will later
be used by fiemap too (patch 10/10). More details in the changelogs.

There are a few more things that can be done to speedup fiemap and lseek,
but I'll leave those other optimizations I have in mind for some other time.

Filipe Manana (10):
  btrfs: allow hole and data seeking to be interruptible
  btrfs: make hole and data seeking a lot more efficient
  btrfs: remove check for impossible block start for an extent map at fiemap
  btrfs: remove zero length check when entering fiemap
  btrfs: properly flush delalloc when entering fiemap
  btrfs: allow fiemap to be interruptible
  btrfs: rename btrfs_check_shared() to a more descriptive name
  btrfs: speedup checking for extent sharedness during fiemap
  btrfs: skip unnecessary extent buffer sharedness checks during fiemap
  btrfs: make fiemap more efficient and accurate reporting extent sharedness

 fs/btrfs/backref.c     | 153 ++++++++-
 fs/btrfs/backref.h     |  20 +-
 fs/btrfs/ctree.h       |  22 +-
 fs/btrfs/extent-tree.c |  10 +-
 fs/btrfs/extent_io.c   | 703 ++++++++++++++++++++++++++++-------------
 fs/btrfs/file.c        | 439 +++++++++++++++++++++++--
 fs/btrfs/inode.c       | 146 ++-------
 7 files changed, 1111 insertions(+), 382 deletions(-)

-- 
2.35.1