[PATCH 5.15 173/183] btrfs: properly report 0 avail for very full file systems

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5.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Josef Bacik <josef@xxxxxxxxxxxxxx>

commit 58bfe2ccec5f9f137b41dd38f335290dcc13cd5c upstream.

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@xxxxxxxxxxxxxxx # 4.14+
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/btrfs/super.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2364,7 +2364,7 @@ static int btrfs_statfs(struct dentry *d
 	 * 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;





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