As Eric reported:

On xfstest generic/204 on f2fs, I'm getting a kernel BUG.

 allocate_segment_by_default+0x9d/0x100 [f2fs]
 f2fs_allocate_data_block+0x3c0/0x5c0 [f2fs]
 do_write_page+0x62/0x110 [f2fs]
 f2fs_do_write_node_page+0x2b/0xa0 [f2fs]
 __write_node_page+0x2ec/0x590 [f2fs]
 f2fs_sync_node_pages+0x756/0x7e0 [f2fs]
 block_operations+0x25b/0x350 [f2fs]
 f2fs_write_checkpoint+0x104/0x1150 [f2fs]
 f2fs_sync_fs+0xa2/0x120 [f2fs]
 f2fs_balance_fs_bg+0x33c/0x390 [f2fs]
 f2fs_write_node_pages+0x4c/0x1f0 [f2fs]
 do_writepages+0x1c/0x70
 __writeback_single_inode+0x45/0x320
 writeback_sb_inodes+0x273/0x5c0
 wb_writeback+0xff/0x2e0
 wb_workfn+0xa1/0x370
 process_one_work+0x138/0x350
 worker_thread+0x4d/0x3d0
 kthread+0x109/0x140

The root cause of this issue is, in a very small partition, e.g.
in generic/204 testcase of fstest suit, filesystem's free space
is 50MB, so at most we can write 12800 inline inode with command:
`echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > $SCRATCH_MNT/$i`,
then filesystem will have:
- 12800 dirty inline data page
- 12800 dirty inode page
- and 12800 dirty imeta (dirty inode)

When we flush node-inode's page cache, we can also flush inline
data with each inode page, however it will run out-of-free-space
in device, then once it triggers checkpoint, there is no room for
huge number of imeta, at this time, GC is useless, as there is no
dirty segment at all.

In order to fix this, we try to recognize inode page during
node_inode's page flushing, and update inode page from dirty inode,
so that later another imeta (dirty inode) flush can be avoided.
Reported-and-tested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>