[PATCH 0/5 v6] ext4: Speedup orphan file handling

  Hello,

Here is a fourth version of my series to speed up orphan inode handling in
ext4.

Orphan inode handling in ext4 is a bottleneck for workloads which heavily
excercise truncate / unlink of small files as they contend on global
s_orphan_mutex (when you have fast enough storage). This patch set implements
new way of handling orphan inodes - instead of using a linked list, we store
inode numbers of orphaned inodes in a file which is possible to implement in a
more scalable manner than linked list manipulations. See description of patch
3/5 for more details.

The patch set achieves significant gains both for a micro benchmark stressing
orphan inode handling (truncating file byte-by-byte, several threads in
parallel) and for reaim creat_clo workload. I'm happy for any review, thoughts,
ideas about the patches. I have also implemented full support in e2fsprogs
which I'll send separately.

								Honza

[1] https://lore.kernel.org/lkml/20210227120804.GB22871@xsang-OptiPlex-9020/ (local)

Changes since v5:
* Added Reviewed-by tags from Ted
* Fixed up sparse warning spotted by 0-day
* Fixed error handling path in ext4_orphan_add() to not leak orphan entry

Changes since v4:
* Rebased on top of v5.14-rc5
* Updated commit message of patch 1/5
* Added Reviewed-by tags from Ted

Changes since v3:
* Added documentation about on-disk format changes
* Add physical block number into orphan block checksum
* Improve some sanity checks, handling of corrupted orphan file
* Improved some changelogs

Changes since v2:
* Updated some comments
* Rebased onto 5.13-rc5
* Change orphan file inode from a fixed inode number to inode number stored
  in the superblock

Changes since v1:
* orphan blocks have now magic numbers
* split out orphan handling to a separate source file
* some smaller updates according to review

Previous versions:
Link: http://lore.kernel.org/r/20210811101006.2033-1-jack@suse.cz (local) # v5
Link: https://lore.kernel.org/linux-ext4/20210712154009.9290-1-jack@suse.cz/ (local) #v4
Link: https://lore.kernel.org/linux-ext4/20210616105655.5129-1-jack@suse.cz/ (local) #v3
Link: https://lore.kernel.org/linux-ext4/1432293717-24010-1-git-send-email-jack@suse.cz/ (local) #v2