In the cache writing process, if it is an atomic file, increase the page
count of F2FS_WB_CP_DATA, otherwise increase the page count of
F2FS_WB_DATA.

When you step into the hook branch due to insufficient memory in
f2fs_write_begin, f2fs_drop_inmem_pages_all will be called to traverse
all atomic inodes and clear the FI_ATOMIC_FILE mark of all atomic files.

In f2fs_drop_inmem_pages，first acquire the inmem_lock , revoke all the
inmem_pages, and then clear the FI_ATOMIC_FILE mark. Before this mark is
cleared, other threads may hold inmem_lock to add inmem_pages to the inode
that has just been emptied inmem_pages, and increase the page count of
F2FS_WB_CP_DATA.

When the IO returns, it is found that the FI_ATOMIC_FILE flag is cleared
by f2fs_drop_inmem_pages_all, and f2fs_is_atomic_file returns false,which
causes the page count of F2FS_WB_DATA to be decremented. The page count of
F2FS_WB_CP_DATA cannot be cleared. Finally, hungtask is triggered in
f2fs_wait_on_all_pages because get_pages will never return zero.

process A:				process B:
f2fs_drop_inmem_pages_all
->f2fs_drop_inmem_pages of inode#1
    ->mutex_lock(&fi->inmem_lock)
    ->__revoke_inmem_pages of inode#1	f2fs_ioc_commit_atomic_write
    ->mutex_unlock(&fi->inmem_lock)	->f2fs_commit_inmem_pages of inode#1
					->mutex_lock(&fi->inmem_lock)
					->__f2fs_commit_inmem_pages
					    ->f2fs_do_write_data_page
					        ->f2fs_outplace_write_data
					            ->do_write_page
					                ->f2fs_submit_page_write
					                    ->inc_page_count(sbi, F2FS_WB_CP_DATA )
					->mutex_unlock(&fi->inmem_lock)
    ->spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
    ->clear_inode_flag(inode, FI_ATOMIC_FILE)
    ->spin_unlock(&sbi->inode_lock[ATOMIC_FILE])
					f2fs_write_end_io
					->dec_page_count(sbi, F2FS_WB_DATA );

We can fix the problem by putting the action of clearing the FI_ATOMIC_FILE
mark into the inmem_lock lock. This operation can ensure that no one will
submit the inmem pages before the FI_ATOMIC_FILE mark is cleared, so that
there will be no atomic writes waiting for writeback.

Fixes: 57864ae5 ("f2fs: limit # of inmemory pages")
Signed-off-by: Yi Zhuang <zhuangyi1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>