Re: [PATCH v8 4/5] xfs: support shrinking unused space in the last AG

[Brian Foster <bfoster@redhat.com>]

On Mon, Mar 22, 2021 at 08:07:22PM +0800, Gao Xiang wrote:
> On Mon, Mar 22, 2021 at 07:30:40AM -0400, Brian Foster wrote:
> > On Fri, Mar 05, 2021 at 10:57:02AM +0800, Gao Xiang wrote:
> > > As the first step of shrinking, this attempts to enable shrinking
> > > unused space in the last allocation group by fixing up freespace
> > > btree, agi, agf and adjusting super block and use a helper
> > > xfs_ag_shrink_space() to fixup the last AG.
> > > 
> > > This can be all done in one transaction for now, so I think no
> > > additional protection is needed.
> > > 
> > > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > > Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> > > ---
> > >  fs/xfs/xfs_fsops.c | 88 ++++++++++++++++++++++++++++------------------
> > >  fs/xfs/xfs_trans.c |  1 -
> > >  2 files changed, 53 insertions(+), 36 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> > > index fc9e799b2ae3..71cba61a451c 100644
> > > --- a/fs/xfs/xfs_fsops.c
> > > +++ b/fs/xfs/xfs_fsops.c
...
> > > @@ -115,10 +120,15 @@ xfs_growfs_data_private(
> > >  	if (nb_mod && nb_mod < XFS_MIN_AG_BLOCKS) {
> > >  		nagcount--;
> > >  		nb = (xfs_rfsblock_t)nagcount * mp->m_sb.sb_agblocks;
> > > -		if (nb < mp->m_sb.sb_dblocks)
> > > -			return -EINVAL;
> > >  	}
> > >  	delta = nb - mp->m_sb.sb_dblocks;
> > > +	/*
> > > +	 * XFS doesn't really support single-AG filesystems, so do not
> > > +	 * permit callers to remove the filesystem's second and last AG.
> > > +	 */
> > > +	if (delta < 0 && nagcount < 2)
> > > +		return -EINVAL;
> > > +
> > 
> > What if the filesystem is already single AG? Unless I'm missing
> > something, we already have a check a bit further down that prevents
> > removal of AGs in the first place.
> 
> I think it tends to forbid (return -EINVAL) shrinking the filesystem with
> a single AG only? Am I missing something?
> 

My assumption was this check means one can't shrink a filesystem that is
already agcount == 1. The comment refers to preventing shrink from
causing an agcount == 1 fs. What is the intent?

Brian

> Thanks,
> Gao Xiang
> 
> > 
> > Otherwise looks reasonable..
> > 
> > Brian
> > 
> > >  	oagcount = mp->m_sb.sb_agcount;
> > >  
> > >  	/* allocate the new per-ag structures */
> > > @@ -126,15 +136,22 @@ xfs_growfs_data_private(
> > >  		error = xfs_initialize_perag(mp, nagcount, &nagimax);
> > >  		if (error)
> > >  			return error;
> > > +	} else if (nagcount < oagcount) {
> > > +		/* TODO: shrinking the entire AGs hasn't yet completed */
> > > +		return -EINVAL;
> > >  	}
> > >  
> > >  	error = xfs_trans_alloc(mp, &M_RES(mp)->tr_growdata,
> > > -			XFS_GROWFS_SPACE_RES(mp), 0, XFS_TRANS_RESERVE, &tp);
> > > +			(delta > 0 ? XFS_GROWFS_SPACE_RES(mp) : -delta), 0,
> > > +			XFS_TRANS_RESERVE, &tp);
> > >  	if (error)
> > >  		return error;
> > >  
> > > -	error = xfs_resizefs_init_new_ags(tp, &id, oagcount, nagcount,
> > > -					  delta, &lastag_resetagres);
> > > +	if (delta > 0)
> > > +		error = xfs_resizefs_init_new_ags(tp, &id, oagcount, nagcount,
> > > +						  delta, &lastag_resetagres);
> > > +	else
> > > +		error = xfs_ag_shrink_space(mp, &tp, nagcount - 1, -delta);
> > >  	if (error)
> > >  		goto out_trans_cancel;
> > >  
> > > @@ -145,7 +162,7 @@ xfs_growfs_data_private(
> > >  	 */
> > >  	if (nagcount > oagcount)
> > >  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_AGCOUNT, nagcount - oagcount);
> > > -	if (delta > 0)
> > > +	if (delta)
> > >  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_DBLOCKS, delta);
> > >  	if (id.nfree)
> > >  		xfs_trans_mod_sb(tp, XFS_TRANS_SB_FDBLOCKS, id.nfree);
> > > @@ -169,28 +186,29 @@ xfs_growfs_data_private(
> > >  	xfs_set_low_space_thresholds(mp);
> > >  	mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
> > >  
> > > -	/*
> > > -	 * If we expanded the last AG, free the per-AG reservation
> > > -	 * so we can reinitialize it with the new size.
> > > -	 */
> > > -	if (lastag_resetagres) {
> > > -		struct xfs_perag	*pag;
> > > -
> > > -		pag = xfs_perag_get(mp, id.agno);
> > > -		error = xfs_ag_resv_free(pag);
> > > -		xfs_perag_put(pag);
> > > -		if (error)
> > > -			return error;
> > > +	if (delta > 0) {
> > > +		/*
> > > +		 * If we expanded the last AG, free the per-AG reservation
> > > +		 * so we can reinitialize it with the new size.
> > > +		 */
> > > +		if (lastag_resetagres) {
> > > +			struct xfs_perag	*pag;
> > > +
> > > +			pag = xfs_perag_get(mp, id.agno);
> > > +			error = xfs_ag_resv_free(pag);
> > > +			xfs_perag_put(pag);
> > > +			if (error)
> > > +				return error;
> > > +		}
> > > +		/*
> > > +		 * Reserve AG metadata blocks. ENOSPC here does not mean there
> > > +		 * was a growfs failure, just that there still isn't space for
> > > +		 * new user data after the grow has been run.
> > > +		 */
> > > +		error = xfs_fs_reserve_ag_blocks(mp);
> > > +		if (error == -ENOSPC)
> > > +			error = 0;
> > >  	}
> > > -
> > > -	/*
> > > -	 * Reserve AG metadata blocks. ENOSPC here does not mean there was a
> > > -	 * growfs failure, just that there still isn't space for new user data
> > > -	 * after the grow has been run.
> > > -	 */
> > > -	error = xfs_fs_reserve_ag_blocks(mp);
> > > -	if (error == -ENOSPC)
> > > -		error = 0;
> > >  	return error;
> > >  
> > >  out_trans_cancel:
> > > diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> > > index 44f72c09c203..d047f5f26cc0 100644
> > > --- a/fs/xfs/xfs_trans.c
> > > +++ b/fs/xfs/xfs_trans.c
> > > @@ -434,7 +434,6 @@ xfs_trans_mod_sb(
> > >  		tp->t_res_frextents_delta += delta;
> > >  		break;
> > >  	case XFS_TRANS_SB_DBLOCKS:
> > > -		ASSERT(delta > 0);
> > >  		tp->t_dblocks_delta += delta;
> > >  		break;
> > >  	case XFS_TRANS_SB_AGCOUNT:
> > > -- 
> > > 2.27.0
> > > 
> > 
>