f2fs: fix to trigger foreground gc during f2fs_map_blocks() in lfs mode

w/ "mode=lfs" mount option, generic/299 will cause system panic as below:

------------[ cut here ]------------
kernel BUG at fs/f2fs/segment.c:2835!
Call Trace:
 <TASK>
 f2fs_allocate_data_block+0x6f4/0xc50
 f2fs_map_blocks+0x970/0x1550
 f2fs_iomap_begin+0xb2/0x1e0
 iomap_iter+0x1d6/0x430
 __iomap_dio_rw+0x208/0x9a0
 f2fs_file_write_iter+0x6b3/0xfa0
 aio_write+0x15d/0x2e0
 io_submit_one+0x55e/0xab0
 __x64_sys_io_submit+0xa5/0x230
 do_syscall_64+0x84/0x2f0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0010:new_curseg+0x70f/0x720

The root cause of we run out-of-space is: in f2fs_map_blocks(), f2fs may
trigger foreground gc only if it allocates any physical block, it will be
a little bit later when there is multiple threads writing data w/
aio/dio/bufio method in parallel, since we always use OPU in lfs mode, so
f2fs_map_blocks() does block allocations aggressively.

In order to fix this issue, let's give a chance to trigger foreground
gc in prior to block allocation in f2fs_map_blocks().

Fixes: 36abef4 ("f2fs: introduce mode=lfs mount option")
Cc: Daeho Jeong <[email protected]>
Signed-off-by: Chao Yu <[email protected]>
Signed-off-by: Jaegeuk Kim <[email protected]>

Author: chaseyu

fs/f2fs/data.c

@@ -1573,8 +1573,11 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map, int flag)
 	end = pgofs + maxblocks;
 
 next_dnode:
-	if (map->m_may_create)
+	if (map->m_may_create) {
+		if (f2fs_lfs_mode(sbi))
+			f2fs_balance_fs(sbi, true);
 		f2fs_map_lock(sbi, flag);
+	}
 
 	/* When reading holes, we need its node page */
 	set_new_dnode(&dn, inode, NULL, NULL, 0);