While reviewing the online fsck patchset, someone spied the
xfs_swapext_can_use_without_log_assistance function and wondered why we
go through this inverted-bitmask dance to avoid setting the
XFS_SB_FEAT_INCOMPAT_LOG_SWAPEXT feature.

(The same principles apply to the logged extended attribute update
feature bit in the since-merged LARP series.)

The reason for this dance is that xfs_add_incompat_log_feature is an
expensive operation -- it forces the log, pushes the AIL, and then if
nobody's beaten us to it, sets the feature bit and issues a synchronous
write of the primary superblock.  That could be a one-time cost
amortized over the life of the filesystem, but the log quiesce and cover
operations call xfs_clear_incompat_log_features to remove feature bits
opportunistically.  On a moderately loaded filesystem this leads to us
cycling those bits on and off over and over, which hurts performance.

Why do we clear the log incompat bits?  Back in ~2020 I think Dave and I
had a conversation on IRC[2] about what the log incompat bits represent.
IIRC in that conversation we decided that the log incompat bits protect
unrecovered log items so that old kernels won't try to recover them and
barf.  Since a clean log has no protected log items, we could clear the
bits at cover/quiesce time.

As Dave Chinner pointed out in the thread, clearing log incompat bits at
unmount time has positive effects for golden root disk image generator
setups, since the generator could be running a newer kernel than what
gets written to the golden image -- if there are log incompat fields set
in the golden image that was generated by a newer kernel/OS image
builder then the provisioning host cannot mount the filesystem even
though the log is clean and recovery is unnecessary to mount the
filesystem.

Given that it's expensive to set log incompat bits, we really only want
to do that once per bit per mount.  Therefore, I propose that we only
clear log incompat bits as part of writing a clean unmount record.  Do
this by adding an operational state flag to the xfs mount that guards
whether or not the feature bit clearing can actually take place.

This eliminates the l_incompat_users rwsem that we use to protect a log
cleaning operation from clearing a feature bit that a frontend thread is
trying to set -- this lock adds another way to fail w.r.t. locking.  For
the swapext series, I shard that into multiple locks just to work around
the lockdep complaints, and that's fugly.

Link: https://lore.kernel.org/linux-xfs/20240131230043.GA6180@frogsfrogsfrogs/Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>