[v2,0/4] btrfs: more explaination on extent_map members

Message

Qu Wenruo April 3, 2024, 11:41 p.m. UTC

[REPO]
https://github.com/adam900710/linux.git/ em_cleanup

[CHANGELOG]
v2:
- Add Filipe's cleanup on mod_start/mod_len
  These two members are no longer utilized, saving me quite some time on
  digging into their usage.

- Update the comments of the extent_map structure
  To make them more readable and less confusing.

- Further cleanup for inline extent_map reading

- A new patch to do extra sanity checks for create_io_em()
  Firstly pure NOCOW writes should not call create_io_em(), secondly
  with the new knowledge of extent_map, it's easier to do extra sanity
  checks for the already pretty long parameter list.

Btrfs uses extent_map to represent a in-memory file extent.

There are severam members that are 1:1 mappe in on-disk file extent
items and extent maps:

- extent_map::start	==	key.offset
- extent_map::len	==	file_extent_num_bytes
- extent_map::ram_bytes	==	file_extent_ram_bytes

But that's all, the remaining are pretty different:

- Use block_start to indicate holes/inline extents
  Meanwhile btrfs on-disk file extent items go with a dedicated type for
  inline extents, and disk_bytenr 0 for holes.

- Extra members for fsync optimization
  I'm still not 100% sure how mod_start and mod_len really works though.

- Weird block_start/orig_block_len/orig_start
  In theory we can directly go with the same file_extent_disk_bytenr,
  file_extent_disk_num_bytes and file_extent_offset to calculate the
  remaining members (block_start/orig_start/orig_block_len/block_len).

  But for whatever reason, we didn't go that path and have a hell of
  weird and inconsistent calculation for them.

I do not have the confidence to handle the mess yet, but as the first
step, I would add comments for those members mostly according to
btrfs_extent_item_to_extent_map(), and hopefully we can improve the
situation in not-far-away future.

Filipe Manana (1):
  btrfs: remove not needed mod_start and mod_len from struct extent_map

Qu Wenruo (3):
  btrfs: add extra comments on extent_map members
  btrfs: simplify the inline extent map creation
  btrfs: add extra sanity checks for create_io_em()

 fs/btrfs/extent_map.c        | 18 -----------
 fs/btrfs/extent_map.h        | 58 ++++++++++++++++++++++++++++++++----
 fs/btrfs/file-item.c         | 15 +++++-----
 fs/btrfs/inode.c             | 44 ++++++++++++++++++++++++---
 fs/btrfs/tree-log.c          |  4 +--
 include/trace/events/btrfs.h |  3 +-
 6 files changed, 104 insertions(+), 38 deletions(-)