btrfs: properly report 0 avail for very full file systems

A user reported some issues with smaller file systems that get very
full.  While investigating this issue I noticed that df wasn't showing
100% full, despite having 0 chunk space and having < 1MiB of available
metadata space.

This turns out to be an overflow issue, we're doing:

  total_available_metadata_space - SZ_4M < global_block_rsv_size

to determine if there's not enough space to make metadata allocations,
which overflows if total_available_metadata_space is < 4M.  Fix this by
checking to see if our available space is greater than the 4M threshold.
This makes df properly report 100% usage on the file system.

CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index cffdd6f..1a093ec 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2117,7 +2117,7 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
         * calculated f_bavail.
         */
        if (!mixed && block_rsv->space_info->full &&
-           total_free_meta - thresh < block_rsv->size)
+           (total_free_meta < thresh || total_free_meta - thresh < block_rsv->size))
                buf->f_bavail = 0;
 
        buf->f_type = BTRFS_SUPER_MAGIC;