[PATCH v2 0/2] btrfs: go extent-by-extent for buffered reads and writes

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v2:
- Fix the length of advancement when no OE is found
  We should still retry the next block, as there may be only a block of
  gap.

  Exposed by Sashiko on the 2nd patch.

  Although it also exposed a false alert on the
  truncate_ordered_extents_beyond_eof().
  Where all the blocks in the range should have an OE, or we're having
  a bigger problem.

Although we have large data folio support for a while, the bufferedL
reads and writes are still iterating a large folio block-by-block.

For a large block sized buffered IO, the large folio has a very high
chance to contain only one single extent.

In that case, although doing block-by-block checks is good for
readability, it's not really performant.

The series changes the behavior to do extent-by-extent iteration
instead, this can bring a very slight performance improve.

For best case scenario, the runtime to submit a folio read can be
reduced from 32us to 2.5us, and a much better distribution.

For writes it's not that obvious for the average runtime, but an obvious
improvment to the distribution.

My current guess for the lack of write performance improment is more
bio_add_folio() failure thus more bio allocations.


Qu Wenruo (2):
  btrfs: read a folio extent-by-extent instead of block-by-block
  btrfs: write back a folio extent-by-extent instead of block-by-block

 fs/btrfs/extent_io.c | 313 +++++++++++++++++++++++++------------------
 fs/btrfs/subpage.c   |  49 +++++++
 fs/btrfs/subpage.h   |   4 +
 3 files changed, 237 insertions(+), 129 deletions(-)

-- 
2.55.0