[RFC PATCH 0/2] xfs: more unlinked inode list optimization v1

[Gao Xiang <hsiangkao@redhat.com>]

Hi forks,

This RFC patchset mainly addresses the thoughts [*] and [**] from Dave's
original patchset,
https://lore.kernel.org/r/20200623095015.1934171-1-david@fromorbit.com

In short, it focues on the following ideas mentioned by Dave:
 - use bucket 0 instead of multiple buckets since in-memory double
   linked list finally works;

 - avoid taking AGI buffer and unnecessary AGI update if possible, so
   1) add a new lock and keep proper locking order to avoid deadlock;
   2) insert a new unlinked inode from the tail instead of head;

In addition, it's worth noticing 3 things:
 - xfs_iunlink_remove() should support old multiple buckets in order
   to keep old inode unlinked list (old image) working when recovering.

 - (but) OTOH, the old kernel recovery _shouldn't_ work with new image
   since the bucket_index from old xfs_iunlink_remove() is generated
   by the old formula (rather than keep in xfs_inode), which is now
   fixed as 0. So this feature is not forward compatible without some
   extra backport patches;

 - a tail xfs_inode pointer is also added in the perag, which keeps 
   track of the tail of bucket 0 since it's mainly used for xfs_iunlink().

 - the old kernel recovery shouldn't work with new image since
     its bucket_index from old kernel is 

The git tree is also available at
git://git.kernel.org/pub/scm/linux/kernel/git/xiang/linux.git tags/xfs/iunlink_opt_v1

Some limited test for debugging only is done (mainly fsstress and
manual power-cut tests), so it could not work as expected just like
my limited broken xfs knowledge. But I will go on improving this
patchset recently. 

Any comments and directions are welcome. :)

Thanks,
Gao Xiang

Gao Xiang (2):
  xfs: arrange all unlinked inodes into one list
  xfs: don't access AGI on unlinked inodes if it can

 fs/xfs/xfs_inode.c       | 283 ++++++++++++++++++++-------------------
 fs/xfs/xfs_log_recover.c |   6 +
 fs/xfs/xfs_mount.c       |   3 +
 fs/xfs/xfs_mount.h       |   3 +
 4 files changed, 160 insertions(+), 135 deletions(-)

-- 
2.18.1