f2fs-for-4.19-rc1 In this round, we've tuned f2fs to improve general performance by serializing block allocation and enhancing discard flows like fstrim which avoids user IO contention. And we've added fsync_mode=nobarrier which gives an option to user where it skips issuing cache_flush commands to underlying flash storage. And there are many bug fixes related to fuzzed images, revoked atomic writes, quota ops, and minor direct IO. Enhancement: - add fsync_mode=nobarrier which bypasses cache_flush command - enhance the discarding flow which avoids user IOs and issues in LBA order - readahead some encrypted blocks during GC - enable in-memory inode checksum to verify the blocks if F2FS_CHECK_FS is set - enhance nat_bits behavior - set -o discard by default - set REQ_RAHEAD to bio in ->readpages Bug fixes: - fix a corner case to corrupt atomic_writes revoking flow - revisit i_gc_rwsem to fix race conditions - fix some dio behaviors captured by xfstests - correct handling errors given by quota-related failures - add many sanity check flows to avoid fuzz test failures - add more error number propagation to their callers - fix several corner cases to continue fault injection w/ shutdown loop