ext4: ensure f_bfree returned by ext4_statfs() is non-negative

I found the issue that the number of free blocks went negative.
# stat -f /mnt/mp1/
File: "/mnt/mp1/"
ID: e175ccb83a872efe Namelen: 255 Type: ext2/ext3
Block size: 4096 Fundamental block size: 4096
Blocks: Total: 258022 Free: -15 Available: -13122
Inodes: Total: 65536 Free: 63029

f_bfree in struct statfs will go negative when the filesystem has
few free blocks. Because the number of dirty blocks is bigger than
the number of free blocks in the following two cases.

CASE 1:
ext4_da_writepages
mpage_da_map_and_submit
ext4_map_blocks
ext4_ext_map_blocks
ext4_mb_new_blocks
ext4_mb_diskspace_used
percpu_counter_sub(&sbi->s_freeblocks_counter, ac->ac_b_ex.fe_len);
<--- interrupt statfs systemcall --->
ext4_da_update_reserve_space
percpu_counter_sub(&sbi->s_dirtyblocks_counter,
used + ei->i_allocated_meta_blocks);

CASE 2:
ext4_write_begin
__block_write_begin
ext4_map_blocks
ext4_ext_map_blocks
ext4_mb_new_blocks
ext4_mb_diskspace_used
percpu_counter_sub(&sbi->s_freeblocks_counter, ac->ac_b_ex.fe_len);
<--- interrupt statfs systemcall --->
percpu_counter_sub(&sbi->s_dirtyblocks_counter, reserv_blks);

To avoid the issue, this patch ensures that f_bfree is non-negative.

Signed-off-by: Kazuya Mio <[e-mail]>

fs/ext4/super.c

···
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_super_block *es = sbi->s_es;
 	u64 fsid;
+	s64 bfree;
 
 	if (test_opt(sb, MINIX_DF)) {
 		sbi->s_overhead_last = 0;
···
 	buf->f_type = EXT4_SUPER_MAGIC;
 	buf->f_bsize = sb->s_blocksize;
 	buf->f_blocks = ext4_blocks_count(es) - sbi->s_overhead_last;
-	buf->f_bfree = percpu_counter_sum_positive(&sbi->s_freeblocks_counter) -
+	bfree = percpu_counter_sum_positive(&sbi->s_freeblocks_counter) -
 		       percpu_counter_sum_positive(&sbi->s_dirtyblocks_counter);
+	/* prevent underflow in case that few free space is available */
+	buf->f_bfree = max_t(s64, bfree, 0);
 	buf->f_bavail = buf->f_bfree - ext4_r_blocks_count(es);
 	if (buf->f_bfree < ext4_r_blocks_count(es))
 		buf->f_bavail = 0;