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

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Mar 21, 2022 at 11:22:58AM -0400, Brian Foster wrote:
> 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! :)

Heh, that was a bad sentence.

I'll update the end of that to read:

"...or the internal per-AG set-aside that prevents unwanted filesystem
shutdowns due to ENOSPC during a bmap btree split."

> > 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.).

Ok.  Added the following at the end of the commit log:

"This makes the numbers that we return via the statfs f_bavail and
f_bfree fields a more conservative estimate of actual free space."

--D

> 
> 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);
> > 
>