BTW, this patch is for v5.5-rc release.

As some GUI programs, e.g nemo from cinnamon, would check statfs result
before doing any write operation, thus could cause false early ENOSPC error.

Thanks,
Qu

On 2020/1/15 上午11:41, Qu Wenruo wrote:
> [BUG]
> When there are a lot of metadata space reserved, e.g. after balancing a
> data block with many extents, vanilla df would report 0 available space.
> 
> [CAUSE]
> btrfs_statfs() would report 0 available space if its metadata space is
> exhausted.
> And the calculation is based on currently reserved space vs on-disk
> available space, with a small headroom as buffer.
> When there is not enough headroom, btrfs_statfs() will report 0
> available space.
> 
> The problem is, since commit ef1317a1b9a3 ("btrfs: do not allow
> reservations if we have pending tickets"), we allow btrfs to over commit
> metadata space, as long as we have enough space to allocate new metadata
> chunks.
> 
> This makes old calculation unreliable and report false 0 available space.
> 
> [FIX]
> Don't do such naive check anymore for btrfs_statfs().
> Also remove the comment about "0 available space when metadata is
> exhausted".
> 
> Please note that, this is a just a quick fix. There are still a lot of
> things to be improved.
> 
> Fixes: ef1317a1b9a3 ("btrfs: do not allow reservations if we have pending tickets")
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/super.c | 21 ---------------------
>  1 file changed, 21 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index f452a94abdc3..ca1a26b3e884 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2018,8 +2018,6 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
>   * algorithm that respects the device sizes and order of allocations.  This is
>   * a close approximation of the actual use but there are other factors that may
>   * change the result (like a new metadata chunk).
> - *
> - * If metadata is exhausted, f_bavail will be 0.
>   */
>  static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  {
> @@ -2034,7 +2032,6 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	unsigned factor = 1;
>  	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
>  	int ret;
> -	u64 thresh = 0;
>  	int mixed = 0;
>  
>  	rcu_read_lock();
> @@ -2089,24 +2086,6 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	buf->f_bavail += div_u64(total_free_data, factor);
>  	buf->f_bavail = buf->f_bavail >> bits;
>  
> -	/*
> -	 * We calculate the remaining metadata space minus global reserve. If
> -	 * this is (supposedly) smaller than zero, there's no space. But this
> -	 * does not hold in practice, the exhausted state happens where's still
> -	 * some positive delta. So we apply some guesswork and compare the
> -	 * delta to a 4M threshold.  (Practically observed delta was ~2M.)
> -	 *
> -	 * We probably cannot calculate the exact threshold value because this
> -	 * depends on the internal reservations requested by various
> -	 * operations, so some operations that consume a few metadata will
> -	 * succeed even if the Avail is zero. But this is better than the other
> -	 * way around.
> -	 */
> -	thresh = SZ_4M;
> -
> -	if (!mixed && total_free_meta - thresh < block_rsv->size)
> -		buf->f_bavail = 0;
> -
>  	buf->f_type = BTRFS_SUPER_MAGIC;
>  	buf->f_bsize = dentry->d_sb->s_blocksize;
>  	buf->f_namelen = BTRFS_NAME_LEN;
>