Re: [PATCH] btrfs: super: Make btrfs_statfs() work with metadata over-commiting

[Josef Bacik <josef@toxicpanda.com>]

On 12/16/19 1:12 AM, Qu Wenruo wrote:
> [BUG]
> There are several reports about vanilla `df` reports no available space,
> while `btrfs filesystem df` is still reporting tons of unallocated
> space.
> 
> https://lore.kernel.org/linux-btrfs/CAJCQCtQEu_+nL_HByAWK2zKfg2Zhpm3Ezto+sA12wwV0iq8Ghg@mail.gmail.com/T/#t
> https://lore.kernel.org/linux-btrfs/CAJCQCtSWW4ageK56PdHtSmgrFrDf_Qy0PbxZ5LSYYCbr3Z10ug@mail.gmail.com/T/#t
> 
> The example output from vanilla `df` would look like:
> Filesystem  Size  Used Avail Use% Mounted on
> /dev/loop0  7.4T  623G     0 100% /media/backup
> 
> [CAUSE]
> There is a special check in btrfs_statfs(), which reset f_bavail:
> 
> 	if (!mixed && total_free_meta - SZ_4M < block_rsv->size)
> 		buf->f_bavail = 0;

Why not just read fs_info->free_chunk_space and take that into account?  The 
point is we want to tell the user there's no room left if we can't allocate a 
new chunk and we only have the global reserve space left.  So just subtract the 
global reserve size from the total f_bavail as long as free_chunk_space is 
sufficient, otherwise fall back to the original calculation.  Thanks,

Josef