Re: [RFC PATCH] xfs_repair: fix rebuilding btree node block less than minrecs

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Jun 09, 2020 at 07:40:53PM +0800, Gao Xiang wrote:
> In production, we found that sometimes xfs_repair phase 5
> rebuilds freespace node block with pointers less than minrecs
> and if we trigger xfs_repair again it would report such
> the following message:
> 
> bad btree nrecs (39, min=40, max=80) in btbno block 0/7882
> 
> The background is that xfs_repair starts to rebuild AGFL
> after the freespace btree is settled in phase 5 so we may
> need to leave necessary room in advance for each btree
> leaves in order to avoid freespace btree split and then
> result in AGFL rebuild fails. The old mathematics uses
> ceil(num_extents / maxrecs) to decide the number of node
> blocks. That would be fine without leaving extra space
> since minrecs = maxrecs / 2 but if some slack was decreased
> from maxrecs, the result would be larger than what is
> expected and cause num_recs_pb less than minrecs, i.e:
> 
> num_extents = 79, adj_maxrecs = 80 - 2 (slack) = 78
> 
> so we'd get
> 
> num_blocks = ceil(79 / 78) = 2,
> num_recs_pb = 79 / 2 = 39, which is less than
> minrecs = 80 / 2 = 40
> 
> OTOH, btree bulk loading code behaves in a different way.
> As in xfs_btree_bload_level_geometry it wrote
> 
> num_blocks = floor(num_extents / maxrecs)
> 
> which will never go below minrecs. And when it goes
> above maxrecs, just increment num_blocks and recalculate
> so we can get the reasonable results.
> 
> In the long term, btree bulk loader will replace the current
> repair code as well as to resolve AGFL dependency issue.
> But we may still want to look for a backportable solution
> for stable versions. Hence, use the same logic to avoid the
> freespace btree minrecs underflow for now.
> 
> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: Eric Sandeen <sandeen@sandeen.net>
> Fixes: 9851fd79bfb1 ("repair: AGFL rebuild fails if btree split required")
> Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
> ---
> not heavy tested yet..
> 
>  repair/phase5.c | 101 +++++++++++++++++++++---------------------------
>  1 file changed, 45 insertions(+), 56 deletions(-)
> 
> diff --git a/repair/phase5.c b/repair/phase5.c
> index abae8a08..997804a5 100644
> --- a/repair/phase5.c
> +++ b/repair/phase5.c
> @@ -348,11 +348,29 @@ finish_cursor(bt_status_t *curs)
>   * failure at runtime. Hence leave a couple of records slack space in
>   * each block to allow immediate modification of the tree without
>   * requiring splits to be done.
> - *
> - * XXX(hch): any reason we don't just look at mp->m_alloc_mxr?
>   */
> -#define XR_ALLOC_BLOCK_MAXRECS(mp, level) \
> -	(libxfs_allocbt_maxrecs((mp), (mp)->m_sb.sb_blocksize, (level) == 0) - 2)
> +static void
> +compute_level_geometry(xfs_mount_t *mp, bt_stat_level_t *lptr,
> +		       uint64_t nr_this_level, bool leaf)

Please don't use structure typedefs here.

Also, please indent the argument list like the rest of xfs code, e.g.

static void
compute_bnobt_geometry(
	struct xfs_mount	*mp,
	struct bt_stat_level	*lptr,

(etc)

> +{
> +	unsigned int		maxrecs = mp->m_alloc_mxr[!leaf];
> +	int			slack = leaf ? 2 : 0;
> +	unsigned int		desired_npb;
> +
> +	desired_npb = max(mp->m_alloc_mnr[!leaf], maxrecs - slack);
> +	lptr->num_recs_tot = nr_this_level;
> +	lptr->num_blocks = max(1ULL, nr_this_level / desired_npb);
> +
> +	lptr->num_recs_pb = nr_this_level / lptr->num_blocks;
> +	lptr->modulo = nr_this_level % lptr->num_blocks;
> +	if (lptr->num_recs_pb > maxrecs || (lptr->num_recs_pb == maxrecs &&
> +			lptr->modulo)) {

Indentation....

	if (lptr->num_recs_pb > maxrecs ||
	    (lptr->num_recs_pb == maxrecs && lptr->modulo)) {
		lptr->num_blocks++;
		...
	}

> +		lptr->num_blocks++;
> +
> +		lptr->num_recs_pb = nr_this_level / lptr->num_blocks;
> +		lptr->modulo = nr_this_level % lptr->num_blocks;
> +	}
> +}
>  
>  /*
>   * this calculates a freespace cursor for an ag.
> @@ -370,6 +388,7 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  	int			i;
>  	int			extents_used;
>  	int			extra_blocks;
> +	uint64_t		old_blocks;
>  	bt_stat_level_t		*lptr;
>  	bt_stat_level_t		*p_lptr;
>  	extent_tree_node_t	*ext_ptr;
> @@ -388,10 +407,7 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  	 * of the tree and set up the cursor for the leaf level
>  	 * (note that the same code is duplicated further down)
>  	 */
> -	lptr->num_blocks = howmany(num_extents, XR_ALLOC_BLOCK_MAXRECS(mp, 0));
> -	lptr->num_recs_pb = num_extents / lptr->num_blocks;
> -	lptr->modulo = num_extents % lptr->num_blocks;
> -	lptr->num_recs_tot = num_extents;
> +	compute_level_geometry(mp, lptr, num_extents, true);
>  	level = 1;
>  
>  #ifdef XR_BLD_FREE_TRACE
> @@ -406,18 +422,12 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  	 * per level is the # of blocks in the level below it
>  	 */
>  	if (lptr->num_blocks > 1)  {
> -		for (; btree_curs->level[level - 1].num_blocks > 1
> -				&& level < XFS_BTREE_MAXLEVELS;
> -				level++)  {
> +		do {
> +			p_lptr = lptr;
>  			lptr = &btree_curs->level[level];
> -			p_lptr = &btree_curs->level[level - 1];
> -			lptr->num_blocks = howmany(p_lptr->num_blocks,
> -					XR_ALLOC_BLOCK_MAXRECS(mp, level));
> -			lptr->modulo = p_lptr->num_blocks
> -					% lptr->num_blocks;
> -			lptr->num_recs_pb = p_lptr->num_blocks
> -					/ lptr->num_blocks;
> -			lptr->num_recs_tot = p_lptr->num_blocks;
> +
> +			compute_level_geometry(mp, lptr,
> +					p_lptr->num_blocks, false);
>  #ifdef XR_BLD_FREE_TRACE
>  			fprintf(stderr, "%s %d %d %d %d %d\n", __func__,
>  					level,
> @@ -426,7 +436,9 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  					lptr->modulo,
>  					lptr->num_recs_tot);
>  #endif
> -		}
> +			level++;
> +		} while (lptr->num_blocks > 1);
> +		ASSERT (level < XFS_BTREE_MAXLEVELS);
>  	}
>  
>  	ASSERT(lptr->num_blocks == 1);
> @@ -496,8 +508,11 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  	 * see if the number of leaf blocks will change as a result
>  	 * of the number of extents changing
>  	 */
> -	if (howmany(num_extents, XR_ALLOC_BLOCK_MAXRECS(mp, 0))
> -			!= btree_curs->level[0].num_blocks)  {
> +	old_blocks = btree_curs->level[0].num_blocks;
> +	compute_level_geometry(mp, &btree_curs->level[0], num_extents, true);
> +	extra_blocks = 0;
> +
> +	if (old_blocks != btree_curs->level[0].num_blocks)  {
>  		/*
>  		 * yes -- recalculate the cursor.  If the number of
>  		 * excess (overallocated) blocks is < xfs_agfl_size/2, we're ok.
> @@ -553,30 +568,20 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  		}
>  
>  		lptr = &btree_curs->level[0];
> -		lptr->num_blocks = howmany(num_extents,
> -					XR_ALLOC_BLOCK_MAXRECS(mp, 0));
> -		lptr->num_recs_pb = num_extents / lptr->num_blocks;
> -		lptr->modulo = num_extents % lptr->num_blocks;
> -		lptr->num_recs_tot = num_extents;
>  		level = 1;
>  
>  		/*
>  		 * if we need more levels, set them up
>  		 */
>  		if (lptr->num_blocks > 1)  {
> -			for (level = 1; btree_curs->level[level-1].num_blocks
> -					> 1 && level < XFS_BTREE_MAXLEVELS;
> -					level++)  {
> -				lptr = &btree_curs->level[level];
> -				p_lptr = &btree_curs->level[level-1];
> -				lptr->num_blocks = howmany(p_lptr->num_blocks,
> -					XR_ALLOC_BLOCK_MAXRECS(mp, level));
> -				lptr->modulo = p_lptr->num_blocks
> -						% lptr->num_blocks;
> -				lptr->num_recs_pb = p_lptr->num_blocks
> -						/ lptr->num_blocks;
> -				lptr->num_recs_tot = p_lptr->num_blocks;
> -			}
> +			do {
> +				p_lptr = lptr;
> +				lptr = &btree_curs->level[level++];
> +
> +				compute_level_geometry(mp, lptr,
> +						p_lptr->num_blocks, false);
> +			} while (lptr->num_blocks > 1);

/me wonders why it's necessary to wrap the do...while in an if test,
as opposed to using a while loop with the if test up front?

--D

> +			ASSERT (level < XFS_BTREE_MAXLEVELS);
>  		}
>  		ASSERT(lptr->num_blocks == 1);
>  		btree_curs->num_levels = level;
> @@ -591,22 +596,6 @@ calculate_freespace_cursor(xfs_mount_t *mp, xfs_agnumber_t agno,
>  
>  		ASSERT(blocks_allocated_total >= blocks_needed);
>  		extra_blocks = blocks_allocated_total - blocks_needed;
> -	} else  {
> -		if (extents_used > 0) {
> -			/*
> -			 * reset the leaf level geometry to account
> -			 * for consumed extents.  we can leave the
> -			 * rest of the cursor alone since the number
> -			 * of leaf blocks hasn't changed.
> -			 */
> -			lptr = &btree_curs->level[0];
> -
> -			lptr->num_recs_pb = num_extents / lptr->num_blocks;
> -			lptr->modulo = num_extents % lptr->num_blocks;
> -			lptr->num_recs_tot = num_extents;
> -		}
> -
> -		extra_blocks = 0;
>  	}
>  
>  	btree_curs->num_tot_blocks = blocks_allocated_pt;
> -- 
> 2.18.1
>