[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@xxxxxxxxxxxxxx/T/#t https://lore.kernel.org/linux-btrfs/CAJCQCtSWW4ageK56PdHtSmgrFrDf_Qy0PbxZ5LSYYCbr3Z10ug@xxxxxxxxxxxxxx/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; This old code from 2016 mostly assumes btrfs won't reserve too much metadata space beyond free metadata space. However since v5.4, we had a rework on metadata space reservation, now we alloc metadata over-commit (reserve more space than we have, as long as we can allocate enough metadata chunks) without really allocating enough metadata chunks. This means block_rsv->size can easily go beyond total_free_meta, which is the unused metadata space, and results f_bavail becomes 0. [FIX] The perfect fix is to modify btrfs_calc_avail_data_space(), to calculate the needed space for metadata chunks, then calculate how many bytes we can allocate from the remaining unallocated space. But that's too complex just for vanilla `df` command. Here we take a shortcut, by excluding the over-committing metadata space from calculated data space. This is far from perfect, but should work good enough (TM). Reported-by: Tomasz Chmielewski <mangoo@xxxxxxxx> Reported-by: Martin Raiber <martin@xxxxxxxxxxxx> Signed-off-by: Qu Wenruo <wqu@xxxxxxxx> --- fs/btrfs/super.c | 48 +++++++++++++++++++++++++++++++----------------- 1 file changed, 31 insertions(+), 17 deletions(-) diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c index f452a94abdc3..99ee370ba99d 100644 --- a/fs/btrfs/super.c +++ b/fs/btrfs/super.c @@ -2029,12 +2029,13 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf) u64 total_used = 0; u64 total_free_data = 0; u64 total_free_meta = 0; + u64 global_rsv_size; int bits = dentry->d_sb->s_blocksize_bits; __be32 *fsid = (__be32 *)fs_info->fs_devices->fsid; unsigned factor = 1; + unsigned meta_factor = 1; struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv; int ret; - u64 thresh = 0; int mixed = 0; rcu_read_lock(); @@ -2057,6 +2058,8 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf) * Metadata in mixed block goup profiles are accounted in data */ if (!mixed && found->flags & BTRFS_BLOCK_GROUP_METADATA) { + meta_factor = btrfs_bg_type_to_factor( + btrfs_metadata_alloc_profile(fs_info)); if (found->flags & BTRFS_BLOCK_GROUP_DATA) mixed = 1; else @@ -2075,9 +2078,10 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf) /* Account global block reserve as used, it's in logical size already */ spin_lock(&block_rsv->lock); + global_rsv_size = block_rsv->size; /* Mixed block groups accounting is not byte-accurate, avoid overflow */ - if (buf->f_bfree >= block_rsv->size >> bits) - buf->f_bfree -= block_rsv->size >> bits; + if (buf->f_bfree >= global_rsv_size >> bits) + buf->f_bfree -= global_rsv_size >> bits; else buf->f_bfree = 0; spin_unlock(&block_rsv->lock); @@ -2087,25 +2091,35 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf) if (ret) return ret; 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.) + * Btrfs metadata can do over-commit, which means we can have way more + * reserved metadata space as long as we can allocate enough meta + * chunks. + * + * The most accurate way to calculate unallocated free data space would + * involve calculating over-commit meta chunks, then calculate + * how many data space we can allocate from the remaining space. * - * 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. + * But that's too expensive, here we just go easy, excluding the over- + * commiting metadata part from f_bavail. */ - thresh = SZ_4M; + if (global_rsv_size > total_free_meta) { + u64 to_exclude = global_rsv_size - total_free_meta; + + /* + * A quick dirty calculation using factor to handle different + * meta/data factors. E.g. for -d SINGLE -m RAID1, we need to + * exclude twice the space. + */ + to_exclude = to_exclude * meta_factor / factor; - if (!mixed && total_free_meta - thresh < block_rsv->size) - buf->f_bavail = 0; + if (buf->f_bavail > to_exclude) + buf->f_bavail -= to_exclude; + else + buf->f_bavail = 0; + } + buf->f_bavail = buf->f_bavail >> bits; buf->f_type = BTRFS_SUPER_MAGIC; buf->f_bsize = dentry->d_sb->s_blocksize; -- 2.24.1
