Re: [PATCH 5/6] xfs: don't report reserved bnobt space as available

[Brian Foster <bfoster@redhat.com>]

On Sun, Mar 20, 2022 at 09:43:55AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> On a modern filesystem, we don't allow userspace to allocate blocks for
> data storage from the per-AG space reservations, the user-controlled
> reservation pool that prevents ENOSPC in the middle of internal
> operations, or the internal per-AG set-aside that prevents ENOSPC.

We can prevent -ENOSPC now? Neat! :)

> Since we now consider free space btree blocks as unavailable for
> allocation for data storage, we shouldn't report those blocks via statfs
> either.
> 

Might be worth a sentence or two that document the (intentional) side
effects of this from a user perspective. I.e., that technically the
presented free space will be a conservative estimate of actual free
space (since allocbt blocks free up as free extents are consumed, etc.).

Otherwise with that sort of commit log tweak:

Reviewed-by: Brian Foster <bfoster@redhat.com>

> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/xfs_fsops.c |    2 +-
>  fs/xfs/xfs_super.c |    3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> 
> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> index 615334e4f689..863e6389c6ff 100644
> --- a/fs/xfs/xfs_fsops.c
> +++ b/fs/xfs/xfs_fsops.c
> @@ -347,7 +347,7 @@ xfs_fs_counts(
>  	cnt->allocino = percpu_counter_read_positive(&mp->m_icount);
>  	cnt->freeino = percpu_counter_read_positive(&mp->m_ifree);
>  	cnt->freedata = percpu_counter_read_positive(&mp->m_fdblocks) -
> -						mp->m_alloc_set_aside;
> +						xfs_fdblocks_unavailable(mp);
>  
>  	spin_lock(&mp->m_sb_lock);
>  	cnt->freertx = mp->m_sb.sb_frextents;
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index d84714e4e46a..54be9d64093e 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -815,7 +815,8 @@ xfs_fs_statfs(
>  	spin_unlock(&mp->m_sb_lock);
>  
>  	/* make sure statp->f_bfree does not underflow */
> -	statp->f_bfree = max_t(int64_t, fdblocks - mp->m_alloc_set_aside, 0);
> +	statp->f_bfree = max_t(int64_t, 0,
> +				fdblocks - xfs_fdblocks_unavailable(mp));
>  	statp->f_bavail = statp->f_bfree;
>  
>  	fakeinos = XFS_FSB_TO_INO(mp, statp->f_bfree);
>