f2fs-for-5.12-rc1 We've added two major features: 1) compression level and 2) checkpoint_merge, in this round. 1) compression level expands 'compress_algorithm' mount option to accept parameter as format of <algorithm>:<level>, by this way, it gives a way to allow user to do more specified config on lz4 and zstd compression level, then f2fs compression can provide higher compress ratio. 2) checkpoint_merge creates a kernel daemon and makes it to merge concurrent checkpoint requests as much as possible to eliminate redundant checkpoint issues. Plus, we can eliminate the sluggish issue caused by slow checkpoint operation when the checkpoint is done in a process context in a cgroup having low i/o budget and cpu shares. Enhancement: - add compress level for lz4 and zstd in mount option - checkpoint_merge mount option - deprecate f2fs_trace_io Bug fix: - flush data when enabling checkpoint back - handle corner cases of mount options - missing ACL update and lock for I_LINKABLE flag - attach FIEMAP_EXTENT_MERGED in f2fs_fiemap - fix potential deadlock in compression flow - fix wrong submit_io condition As usual, we've cleaned up many code flows and fixed minor bugs.