[v5,0/3] btrfs: more explaination on extent_map members

Message

Qu Wenruo April 15, 2024, 11:35 p.m. UTC

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

The first 3 patches only. As later patches are the huge rename part.

[CHANGELOG]
v5:
- Update the comment on extent map merging part

v4:
- Add extra comments for the extent map merging
  Since extent maps can be merged, the members are only matching
  on-disk file extent items before merging.
  This also means users of extent_map should not rely on it for
  a reliable file extent item member.
  (That's also why we use ordered_extent not extent_maps, to update
   file extent items)

v3:
- Rebased to latest for-next branch
- Further comments polishment
- Coding style update to follow the guideline

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.

- 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.


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.h | 56 ++++++++++++++++++++++++++++++++++++++++++-
 fs/btrfs/file-item.c  | 20 ++++++++--------
 fs/btrfs/inode.c      | 40 ++++++++++++++++++++++++++++++-
 3 files changed, 104 insertions(+), 12 deletions(-)