Re: [PATCH v3 2/2] xfs: set aside allocation btree blocks from block reservation

[Brian Foster <bfoster@redhat.com>]

On Fri, Mar 26, 2021 at 06:34:22PM -0700, Darrick J. Wong wrote:
> On Fri, Mar 19, 2021 at 11:00:29AM -0400, Brian Foster wrote:
> > On Thu, Mar 18, 2021 at 01:31:53PM -0700, Darrick J. Wong wrote:
> > > On Thu, Mar 18, 2021 at 12:17:07PM -0400, Brian Foster wrote:
> > > > The blocks used for allocation btrees (bnobt and countbt) are
> > > > technically considered free space. This is because as free space is
> > > > used, allocbt blocks are removed and naturally become available for
> > > > traditional allocation. However, this means that a significant
> > > > portion of free space may consist of in-use btree blocks if free
> > > > space is severely fragmented.
> > > > 
> > > > On large filesystems with large perag reservations, this can lead to
> > > > a rare but nasty condition where a significant amount of physical
> > > > free space is available, but the majority of actual usable blocks
> > > > consist of in-use allocbt blocks. We have a record of a (~12TB, 32
> > > > AG) filesystem with multiple AGs in a state with ~2.5GB or so free
> > > > blocks tracked across ~300 total allocbt blocks, but effectively at
> > > > 100% full because the the free space is entirely consumed by
> > > > refcountbt perag reservation.
> > > > 
> > > > Such a large perag reservation is by design on large filesystems.
> > > > The problem is that because the free space is so fragmented, this AG
> > > > contributes the 300 or so allocbt blocks to the global counters as
> > > > free space. If this pattern repeats across enough AGs, the
> > > > filesystem lands in a state where global block reservation can
> > > > outrun physical block availability. For example, a streaming
> > > > buffered write on the affected filesystem continues to allow delayed
> > > > allocation beyond the point where writeback starts to fail due to
> > > > physical block allocation failures. The expected behavior is for the
> > > > delalloc block reservation to fail gracefully with -ENOSPC before
> > > > physical block allocation failure is a possibility.
> > > > 
> > > > To address this problem, introduce an in-core counter to track the
> > > > sum of all allocbt blocks in use by the filesystem. Use the new
> > > > counter to set these blocks aside at reservation time and thus
> > > > ensure they cannot be reserved until truly available. Since this is
> > > > only necessary when perag reservations are active and the counter
> > > > requires a read of each AGF to fully populate, only enforce on perag
> > > > res enabled filesystems. This allows initialization of the counter
> > > > at ->pagf_init time because the perag reservation init code reads
> > > > each AGF at mount time.
> > > > 
> > > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > > ---
> > > >  fs/xfs/libxfs/xfs_alloc.c       | 12 ++++++++++++
> > > >  fs/xfs/libxfs/xfs_alloc_btree.c |  2 ++
> > > >  fs/xfs/xfs_mount.c              | 18 +++++++++++++++++-
> > > >  fs/xfs/xfs_mount.h              |  6 ++++++
> > > >  4 files changed, 37 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > > > index 0c623d3c1036..9fa378d2724e 100644
> > > > --- a/fs/xfs/libxfs/xfs_alloc.c
> > > > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > > > @@ -3036,6 +3036,7 @@ xfs_alloc_read_agf(
> > > >  	struct xfs_agf		*agf;		/* ag freelist header */
> > > >  	struct xfs_perag	*pag;		/* per allocation group data */
> > > >  	int			error;
> > > > +	uint32_t		allocbt_blks;
> > > >  
> > > >  	trace_xfs_alloc_read_agf(mp, agno);
> > > >  
> > > > @@ -3066,6 +3067,17 @@ xfs_alloc_read_agf(
> > > >  		pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
> > > >  		pag->pagf_init = 1;
> > > >  		pag->pagf_agflreset = xfs_agfl_needs_reset(mp, agf);
> > > > +
> > > > +		/*
> > > > +		 * Update the global in-core allocbt block counter. Filter
> > > > +		 * rmapbt blocks from the on-disk counter because those are
> > > > +		 * managed by perag reservation.
> > > > +		 */
> > > > +		if (pag->pagf_btreeblks > be32_to_cpu(agf->agf_rmap_blocks)) {
> > > > +			allocbt_blks = pag->pagf_btreeblks -
> > > > +					be32_to_cpu(agf->agf_rmap_blocks);
> > > > +			atomic64_add(allocbt_blks, &mp->m_allocbt_blks);
> > > 
> > > Does growfs call xfs_alloc_read_agf to set pagf_init in the perag
> > > structure when it adds AGs to the filesystem?  I don't /think/ that's
> > > a problem for this use case (since allocbt_blks should be 0 on a freshly
> > > initialized AG) but i was a little surprised to see block reservation
> > > bits here.
> > > 
> > 
> > I'm not sure it matters who reads AGFs as long as the global counter
> > remains consistent. For growing an existing AG, it looks like we "free"
> > the new space into the AG so I think that should be tracked accordingly
> > like any other alloc/free. For a new AG, it looks like ->agf_btreeblks
> > starts at 0 and then the perags would likely init through the perag res
> > call that runs as the final step before the growfs returns.
> 
> <nod>
> 
> > > The other thing is that online repair (what little of it is in the
> > > kernel currently) can set pagf_init = 0; is that a problem?
> > > 
> > 
> > Hmm.. if the AGF is corrupt, it seems likely that the in-core counter is
> > busted as well. We could do something like account the delta from pre
> > and post repair into the global counter, but I'd be weary of scenarios
> > where the AGF might have become inconsistent with the counter somehow
> 
> How would the allocbt_blks counter become inconsistent with the AGF?  We
> update that incore counter at the same time that we update all the other
> ondisk and pag counters, so unless the fs is shut down, we know that
> m_allocbt_blks is off by the same amount as the inconsistent AGF.
> 

Dunno, bit flip or something? Bug? I'm not going to try and predict how
things might break.

> > and the delta itself would throw it off. That might be unlikely, but
> > what scares me about that is we could actually break the global counter
> > by attempting to fix it so allocbt blocks become reservable again.
> 
> But if you don't even /try/ to fix the counter during an AGF repair,
> that almost guarantees that the decisions based on the counter will not
> be correct...
> 

It's not clear to me how to fix the counter after a single AGF repair
such that it's more likely to be correct than not. How do we currently
deal with ->m_fdblocks after a repair of a corrupted AGF? It looks to me
that we mark the summary counters sick and expect them to reinit on the
next mount based on the perag values, but I could be missing something
deeper in the scrub code. Is there a more explicit reinit or something
somewhere?

> > I'm not sure there's a great answer here. Perhaps the safest thing would
> > be to warn about an ->agf_btreeblks inconsistency that might result in
> > some number of "unusable" blocks until a mount cycle occurs and
> > resynchronizes the global counter..? That also seems to be consistent
> > with how we handle the superblock counters after an agf repair.
> 
> ...but OTOH I guess the worst that happens is that we'll ENOSPC early?
> 

Yeah. I think the characteristics of an inconsistent allocbt counter are
either that the set aside is ineffective (i.e. counter too low) or
premature -ENOSPC (counter too high).

Brian

> --D
> 
> > (All that said, I am somewhat hesitant to add to this unless we
> > determine the approach is still viable after the expected perag res
> > changes...).
> > 
> > Brian
> > 
> > > --D
> > > 
> > > > +		}
> > > >  	}
> > > >  #ifdef DEBUG
> > > >  	else if (!XFS_FORCED_SHUTDOWN(mp)) {
> > > > diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
> > > > index 8e01231b308e..9f5a45f7baed 100644
> > > > --- a/fs/xfs/libxfs/xfs_alloc_btree.c
> > > > +++ b/fs/xfs/libxfs/xfs_alloc_btree.c
> > > > @@ -71,6 +71,7 @@ xfs_allocbt_alloc_block(
> > > >  		return 0;
> > > >  	}
> > > >  
> > > > +	atomic64_inc(&cur->bc_mp->m_allocbt_blks);
> > > >  	xfs_extent_busy_reuse(cur->bc_mp, cur->bc_ag.agno, bno, 1, false);
> > > >  
> > > >  	xfs_trans_agbtree_delta(cur->bc_tp, 1);
> > > > @@ -95,6 +96,7 @@ xfs_allocbt_free_block(
> > > >  	if (error)
> > > >  		return error;
> > > >  
> > > > +	atomic64_dec(&cur->bc_mp->m_allocbt_blks);
> > > >  	xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
> > > >  			      XFS_EXTENT_BUSY_SKIP_DISCARD);
> > > >  	xfs_trans_agbtree_delta(cur->bc_tp, -1);
> > > > diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> > > > index 1c97b155a8ee..29f97e909607 100644
> > > > --- a/fs/xfs/xfs_mount.c
> > > > +++ b/fs/xfs/xfs_mount.c
> > > > @@ -1176,6 +1176,7 @@ xfs_mod_fdblocks(
> > > >  	int64_t			lcounter;
> > > >  	long long		res_used;
> > > >  	s32			batch;
> > > > +	uint64_t		set_aside;
> > > >  
> > > >  	if (delta > 0) {
> > > >  		/*
> > > > @@ -1215,8 +1216,23 @@ xfs_mod_fdblocks(
> > > >  	else
> > > >  		batch = XFS_FDBLOCKS_BATCH;
> > > >  
> > > > +	/*
> > > > +	 * Set aside allocbt blocks because these blocks are tracked as free
> > > > +	 * space but not available for allocation. Technically this means that a
> > > > +	 * single reservation cannot consume all remaining free space, but the
> > > > +	 * ratio of allocbt blocks to usable free blocks should be rather small.
> > > > +	 * The tradeoff without this is that filesystems that maintain high
> > > > +	 * perag block reservations can over reserve physical block availability
> > > > +	 * and fail physical allocation, which leads to much more serious
> > > > +	 * problems (i.e. transaction abort, pagecache discards, etc.) than
> > > > +	 * slightly premature -ENOSPC.
> > > > +	 */
> > > > +	set_aside = mp->m_alloc_set_aside;
> > > > +	if (mp->m_has_agresv)
> > > > +		set_aside += atomic64_read(&mp->m_allocbt_blks);
> > > > +
> > > >  	percpu_counter_add_batch(&mp->m_fdblocks, delta, batch);
> > > > -	if (__percpu_counter_compare(&mp->m_fdblocks, mp->m_alloc_set_aside,
> > > > +	if (__percpu_counter_compare(&mp->m_fdblocks, set_aside,
> > > >  				     XFS_FDBLOCKS_BATCH) >= 0) {
> > > >  		/* we had space! */
> > > >  		return 0;
> > > > diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> > > > index 489d9b2c53d9..041f437dc117 100644
> > > > --- a/fs/xfs/xfs_mount.h
> > > > +++ b/fs/xfs/xfs_mount.h
> > > > @@ -171,6 +171,12 @@ typedef struct xfs_mount {
> > > >  	 * extents or anything related to the rt device.
> > > >  	 */
> > > >  	struct percpu_counter	m_delalloc_blks;
> > > > +	/*
> > > > +	 * Global count of allocation btree blocks in use across all AGs. Only
> > > > +	 * used when perag reservation is enabled. Helps prevent block
> > > > +	 * reservation from attempting to reserve allocation btree blocks.
> > > > +	 */
> > > > +	atomic64_t		m_allocbt_blks;
> > > >  
> > > >  	struct radix_tree_root	m_perag_tree;	/* per-ag accounting info */
> > > >  	spinlock_t		m_perag_lock;	/* lock for m_perag_tree */
> > > > -- 
> > > > 2.26.2
> > > > 
> > > 
> > 
>