Re: [PATCH 08/47] xfs: support btrees with overlapping intervals for keys

[Brian Foster <bfoster@redhat.com>]

On Wed, Jul 20, 2016 at 09:56:52PM -0700, Darrick J. Wong wrote:
> On a filesystem with both reflink and reverse mapping enabled, it's
> possible to have multiple rmap records referring to the same blocks on
> disk.  When overlapping intervals are possible, querying a classic
> btree to find all records intersecting a given interval is inefficient
> because we cannot use the left side of the search interval to filter
> out non-matching records the same way that we can use the existing
> btree key to filter out records coming after the right side of the
> search interval.  This will become important once we want to use the
> rmap btree to rebuild BMBTs, or implement the (future) fsmap ioctl.
> 
> (For the non-overlapping case, we can perform such queries trivially
> by starting at the left side of the interval and walking the tree
> until we pass the right side.)
> 
> Therefore, extend the btree code to come closer to supporting
> intervals as a first-class record attribute.  This involves widening
> the btree node's key space to store both the lowest key reachable via
> the node pointer (as the btree does now) and the highest key reachable
> via the same pointer and teaching the btree modifying functions to
> keep the highest-key records up to date.
> 
> This behavior can be turned on via a new btree ops flag so that btrees
> that cannot store overlapping intervals don't pay the overhead costs
> in terms of extra code and disk format changes.
> 
> v2: When we're deleting a record in a btree that supports overlapped
> interval records and the deletion results in two btree blocks being
> joined, we defer updating the high/low keys until after all possible
> joining (at higher levels in the tree) have finished.  At this point,
> the btree pointers at all levels have been updated to remove the empty
> blocks and we can update the low and high keys.
> 
> When we're doing this, we must be careful to update the keys of all
> node pointers up to the root instead of stopping at the first set of
> keys that don't need updating.  This is because it's possible for a
> single deletion to cause joining of multiple levels of tree, and so
> we need to update everything going back to the root.
> 
> v3: Make diff_two_keys return < 0, 0, or > 0 if key1 is less than,
> equal to, or greater than key2, respectively.  This is consistent
> with the rest of the kernel and the C library.  Clarify some comments
> and refactor the sibling_update function out of existence.  Check the
> return value of btree_updkeys().
> 
> v4: In btree_updkeys(), evaluate the force_all parameter before
> running the key diff to avoid reading uninitialized memory when we're
> forcing a key update.  This happens when we've allocated an empty slot
> at level N + 1 to point to a new block at level N and we're in the
> process of filling out the new keys.
> 
> v5: Move the overlapping flag to bc_flags, refactor the get/update keys
> code, and let client btrees set a doublewide key length.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_btree.c |  339 +++++++++++++++++++++++++++++++++++++++++++--
>  fs/xfs/libxfs/xfs_btree.h |   30 ++++
>  fs/xfs/xfs_trace.h        |   36 +++++
>  3 files changed, 392 insertions(+), 13 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index 70d1c60..1881536 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
...
> @@ -1918,14 +2053,107 @@ xfs_btree_get_keys(
>  /*
>   * Decide if we need to update the parent keys of a btree block.  For
>   * a standard btree this is only necessary if we're updating the first
> - * record/key.
> + * record/key.  For an overlapping btree, we must always update the
> + * keys because the highest key can be in any of the records or keys
> + * in the block.
>   */
>  static inline bool
>  xfs_btree_needs_key_update(
>  	struct xfs_btree_cur	*cur,
>  	int			ptr)
>  {
> -	return ptr == 1;
> +	return (cur->bc_flags & XFS_BTREE_OVERLAPPING) || ptr == 1;
> +}
> +
> +/*
> + * Update the low and high parent keys of the given level, progressing
> + * towards the root.  If force_all is false, stop if the keys for a given
> + * level do not need updating.
> + */
> +STATIC int
> +__xfs_btree_updkeys(
> +	struct xfs_btree_cur	*cur,
> +	int			level,
> +	struct xfs_btree_block	*block,
> +	struct xfs_buf		*bp0,
> +	bool			force_all)
> +{
> +	union xfs_btree_bigkey	key;	/* keys from current level */
> +	union xfs_btree_key	*lkey;	/* keys from the next level up */
> +	union xfs_btree_key	*hkey;
> +	union xfs_btree_key	*nlkey;	/* keys from the next level up */
> +	union xfs_btree_key	*nhkey;
> +	struct xfs_buf		*bp;
> +	int			ptr;
> +
> +	ASSERT(cur->bc_flags & XFS_BTREE_OVERLAPPING);
> +
> +	/* Exit if there aren't any parent levels to update. */
> +	if (level + 1 >= cur->bc_nlevels)
> +		return 0;
> +
> +	trace_xfs_btree_updkeys(cur, level, bp0);
> +
> +	lkey = (union xfs_btree_key *)&key;
> +	hkey = xfs_btree_high_key_from_key(cur, lkey);

So we create a bigkey object on the stack which presumably will contain
storage for both keys for overlapping trees, then cast and pass down the
normal xfs_btree_key structure throughout btree infrastructure.

This seems slightly hairy/unfortunate in that we treat in-memory objects
sort of like a buffer pointer. As hch points out, it is also kind of
confusing (I had to skip forward to where the bigkey union was filled in
to get an idea of what was going on). I'm not a huge fan, though I
suppose the alternative might involve changing xfs_btree_key and
possibly a bunch of other code to accommodate the dual keys model.
Perhaps this is in fact the better option.. but have you considered any
options enough to comment on that at all?

> +	xfs_btree_get_keys(cur, block, lkey);
> +	for (level++; level < cur->bc_nlevels; level++) {
> +#ifdef DEBUG
> +		int		error;
> +#endif
> +		block = xfs_btree_get_block(cur, level, &bp);
> +		trace_xfs_btree_updkeys(cur, level, bp);
> +#ifdef DEBUG
> +		error = xfs_btree_check_block(cur, block, level, bp);
> +		if (error) {
> +			XFS_BTREE_TRACE_CURSOR(cur, XBT_ERROR);
> +			return error;
> +		}
> +#endif
> +		ptr = cur->bc_ptrs[level];
> +		nlkey = xfs_btree_key_addr(cur, ptr, block);
> +		nhkey = xfs_btree_high_key_addr(cur, ptr, block);
> +		if (!force_all &&
> +		    !(cur->bc_ops->diff_two_keys(cur, nlkey, lkey) != 0 ||
> +		      cur->bc_ops->diff_two_keys(cur, nhkey, hkey) != 0))
> +			break;
> +		xfs_btree_copy_keys(cur, nlkey, lkey, 1);
> +		xfs_btree_log_keys(cur, bp, ptr, ptr);
> +		if (level + 1 >= cur->bc_nlevels)
> +			break;
> +		cur->bc_ops->get_node_keys(cur, block, lkey);
> +	}
> +
> +	return 0;
> +}
> +
...
>  }
>  
>  /*
...
> @@ -2197,10 +2429,33 @@ xfs_btree_lshift(
>  		rkp = &key;
>  	}
>  
> +	/*
> +	 * Using a temporary cursor, update the parent key values of the
> +	 * block on the left.
> +	 */
> +	error = xfs_btree_dup_cursor(cur, &tcur);
> +	if (error)
> +		goto error0;
> +	i = xfs_btree_firstrec(tcur, level);
> +	XFS_WANT_CORRUPTED_GOTO(cur->bc_mp, i == 1, error0);

Nit:				tcur

> +
> +	error = xfs_btree_decrement(tcur, level, &i);
> +	if (error)
> +		goto error1;
> +

Can we conditionalize tcur creation/processing against the overlapping
flag as well and combine the two associated hunks?

Brian

>  	/* Update the parent keys of the right block. */
>  	error = cur->bc_ops->update_keys(cur, level);
>  	if (error)
> -		goto error0;
> +		goto error1;
> +
> +	/* Update the parent high keys of the left block, if needed. */
> +	if (tcur->bc_flags & XFS_BTREE_OVERLAPPING) {
> +		error = tcur->bc_ops->update_keys(tcur, level);
> +		if (error)
> +			goto error1;
> +	}
> +
> +	xfs_btree_del_cursor(tcur, XFS_BTREE_NOERROR);
>  
>  	/* Slide the cursor value left one. */
>  	cur->bc_ptrs[level]--;
> @@ -2217,6 +2472,11 @@ out0:
>  error0:
>  	XFS_BTREE_TRACE_CURSOR(cur, XBT_ERROR);
>  	return error;
> +
> +error1:
> +	XFS_BTREE_TRACE_CURSOR(tcur, XBT_ERROR);
> +	xfs_btree_del_cursor(tcur, XFS_BTREE_ERROR);
> +	return error;
>  }
>  
>  /*
> @@ -2369,6 +2629,13 @@ xfs_btree_rshift(
>  	if (error)
>  		goto error1;
>  
> +	/* Update the parent high keys of the left block, if needed. */
> +	if (cur->bc_flags & XFS_BTREE_OVERLAPPING) {
> +		error = cur->bc_ops->update_keys(cur, level);
> +		if (error)
> +			goto error1;
> +	}
> +
>  	/* Update the parent keys of the right block. */
>  	error = cur->bc_ops->update_keys(tcur, level);
>  	if (error)
> @@ -2549,6 +2816,14 @@ __xfs_btree_split(
>  		xfs_btree_set_sibling(cur, rrblock, &rptr, XFS_BB_LEFTSIB);
>  		xfs_btree_log_block(cur, rrbp, XFS_BB_LEFTSIB);
>  	}
> +
> +	/* Update the parent high keys of the left block, if needed. */
> +	if (cur->bc_flags & XFS_BTREE_OVERLAPPING) {
> +		error = cur->bc_ops->update_keys(cur, level);
> +		if (error)
> +			goto error0;
> +	}
> +
>  	/*
>  	 * If the cursor is really in the right block, move it there.
>  	 * If it's just pointing past the last entry in left, then we'll
> @@ -2989,7 +3264,8 @@ xfs_btree_insrec(
>  	struct xfs_buf		*bp;	/* buffer for block */
>  	union xfs_btree_ptr	nptr;	/* new block ptr */
>  	struct xfs_btree_cur	*ncur;	/* new btree cursor */
> -	union xfs_btree_key	nkey;	/* new block key */
> +	union xfs_btree_bigkey	nkey;	/* new block key */
> +	union xfs_btree_key	*lkey;
>  	int			optr;	/* old key/record index */
>  	int			ptr;	/* key/record index */
>  	int			numrecs;/* number of records */
> @@ -2997,11 +3273,13 @@ xfs_btree_insrec(
>  #ifdef DEBUG
>  	int			i;
>  #endif
> +	xfs_daddr_t		old_bn;
>  
>  	XFS_BTREE_TRACE_CURSOR(cur, XBT_ENTRY);
>  	XFS_BTREE_TRACE_ARGIPR(cur, level, *ptrp, &rec);
>  
>  	ncur = NULL;
> +	lkey = (union xfs_btree_key *)&nkey;
>  
>  	/*
>  	 * If we have an external root pointer, and we've made it to the
> @@ -3030,6 +3308,7 @@ xfs_btree_insrec(
>  
>  	/* Get pointers to the btree buffer and block. */
>  	block = xfs_btree_get_block(cur, level, &bp);
> +	old_bn = bp ? bp->b_bn : XFS_BUF_DADDR_NULL;
>  	numrecs = xfs_btree_get_numrecs(block);
>  
>  #ifdef DEBUG
> @@ -3056,7 +3335,7 @@ xfs_btree_insrec(
>  	xfs_btree_set_ptr_null(cur, &nptr);
>  	if (numrecs == cur->bc_ops->get_maxrecs(cur, level)) {
>  		error = xfs_btree_make_block_unfull(cur, level, numrecs,
> -					&optr, &ptr, &nptr, &ncur, &nkey, stat);
> +					&optr, &ptr, &nptr, &ncur, lkey, stat);
>  		if (error || *stat == 0)
>  			goto error0;
>  	}
> @@ -3141,8 +3420,17 @@ xfs_btree_insrec(
>  	/* Log the new number of records in the btree header. */
>  	xfs_btree_log_block(cur, bp, XFS_BB_NUMRECS);
>  
> -	/* If we inserted at the start of a block, update the parents' keys. */
> -	if (xfs_btree_needs_key_update(cur, optr)) {
> +	/*
> +	 * If we just inserted into a new tree block, we have to
> +	 * recalculate nkey here because nkey is out of date.
> +	 *
> +	 * Otherwise we're just updating an existing block (having shoved
> +	 * some records into the new tree block), so use the regular key
> +	 * update mechanism.
> +	 */
> +	if (bp && bp->b_bn != old_bn) {
> +		xfs_btree_get_keys(cur, block, lkey);
> +	} else if (xfs_btree_needs_key_update(cur, optr)) {
>  		error = cur->bc_ops->update_keys(cur, level);
>  		if (error)
>  			goto error0;
> @@ -3163,7 +3451,7 @@ xfs_btree_insrec(
>  	 */
>  	*ptrp = nptr;
>  	if (!xfs_btree_ptr_is_null(cur, &nptr)) {
> -		xfs_btree_copy_keys(cur, key, &nkey, 1);
> +		xfs_btree_copy_keys(cur, key, lkey, 1);
>  		*curp = ncur;
>  	}
>  
> @@ -3194,18 +3482,20 @@ xfs_btree_insert(
>  	union xfs_btree_ptr	nptr;	/* new block number (split result) */
>  	struct xfs_btree_cur	*ncur;	/* new cursor (split result) */
>  	struct xfs_btree_cur	*pcur;	/* previous level's cursor */
> -	union xfs_btree_key	key;	/* key of block to insert */
> +	union xfs_btree_bigkey	bkey;	/* key of block to insert */
> +	union xfs_btree_key	*key;
>  	union xfs_btree_rec	rec;	/* record to insert */
>  
>  	level = 0;
>  	ncur = NULL;
>  	pcur = cur;
> +	key = (union xfs_btree_key *)&bkey;
>  
>  	xfs_btree_set_ptr_null(cur, &nptr);
>  
>  	/* Make a key out of the record data to be inserted, and save it. */
>  	cur->bc_ops->init_rec_from_cur(cur, &rec);
> -	cur->bc_ops->init_key_from_rec(&key, &rec);
> +	cur->bc_ops->init_key_from_rec(key, &rec);
>  
>  	/*
>  	 * Loop going up the tree, starting at the leaf level.
> @@ -3217,7 +3507,7 @@ xfs_btree_insert(
>  		 * Insert nrec/nptr into this level of the tree.
>  		 * Note if we fail, nptr will be null.
>  		 */
> -		error = xfs_btree_insrec(pcur, level, &nptr, &rec, &key,
> +		error = xfs_btree_insrec(pcur, level, &nptr, &rec, key,
>  				&ncur, &i);
>  		if (error) {
>  			if (pcur != cur)
> @@ -3916,6 +4206,16 @@ xfs_btree_delrec(
>  	if (level > 0)
>  		cur->bc_ptrs[level]--;
>  
> +	/*
> +	 * We combined blocks, so we have to update the parent keys if the
> +	 * btree supports overlapped intervals.  However, bc_ptrs[level + 1]
> +	 * points to the old block so that the caller knows which record to
> +	 * delete.  Therefore, the caller must be savvy enough to call updkeys
> +	 * for us if we return stat == 2.  The other exit points from this
> +	 * function don't require deletions further up the tree, so they can
> +	 * call updkeys directly.
> +	 */
> +
>  	XFS_BTREE_TRACE_CURSOR(cur, XBT_EXIT);
>  	/* Return value means the next level up has something to do. */
>  	*stat = 2;
> @@ -3941,6 +4241,7 @@ xfs_btree_delete(
>  	int			error;	/* error return value */
>  	int			level;
>  	int			i;
> +	bool			joined = false;
>  
>  	XFS_BTREE_TRACE_CURSOR(cur, XBT_ENTRY);
>  
> @@ -3954,6 +4255,18 @@ xfs_btree_delete(
>  		error = xfs_btree_delrec(cur, level, &i);
>  		if (error)
>  			goto error0;
> +		if (i == 2)
> +			joined = true;
> +	}
> +
> +	/*
> +	 * If we combined blocks as part of deleting the record, delrec won't
> +	 * have updated the parent high keys so we have to do that here.
> +	 */
> +	if (joined && (cur->bc_flags & XFS_BTREE_OVERLAPPING)) {
> +		error = xfs_btree_updkeys_force(cur, 0);
> +		if (error)
> +			goto error0;
>  	}
>  
>  	if (i == 0) {
> diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> index bb40457..3645d91 100644
> --- a/fs/xfs/libxfs/xfs_btree.h
> +++ b/fs/xfs/libxfs/xfs_btree.h
> @@ -44,6 +44,20 @@ union xfs_btree_key {
>  	xfs_inobt_key_t		inobt;
>  };
>  
> +/*
> + * In-core key that holds both low and high keys for overlapped btrees.
> + * The two keys are packed next to each other on disk, so do the same
> + * in memory.  Preserve the existing xfs_btree_key as a single key to
> + * avoid the mental model breakage that would happen if we passed a
> + * bigkey into a function that operates on a single key.
> + */
> +union xfs_btree_bigkey {
> +	struct xfs_bmbt_key		bmbt;
> +	xfs_bmdr_key_t			bmbr;	/* bmbt root block */
> +	xfs_alloc_key_t			alloc;
> +	struct xfs_inobt_key		inobt;
> +};
> +
>  union xfs_btree_rec {
>  	xfs_bmbt_rec_t		bmbt;
>  	xfs_bmdr_rec_t		bmbr;	/* bmbt root block */
> @@ -162,11 +176,21 @@ struct xfs_btree_ops {
>  				     union xfs_btree_rec *rec);
>  	void	(*init_ptr_from_cur)(struct xfs_btree_cur *cur,
>  				     union xfs_btree_ptr *ptr);
> +	void	(*init_high_key_from_rec)(union xfs_btree_key *key,
> +					  union xfs_btree_rec *rec);
>  
>  	/* difference between key value and cursor value */
>  	__int64_t (*key_diff)(struct xfs_btree_cur *cur,
>  			      union xfs_btree_key *key);
>  
> +	/*
> +	 * Difference between key2 and key1 -- positive if key1 > key2,
> +	 * negative if key1 < key2, and zero if equal.
> +	 */
> +	__int64_t (*diff_two_keys)(struct xfs_btree_cur *cur,
> +				   union xfs_btree_key *key1,
> +				   union xfs_btree_key *key2);
> +
>  	const struct xfs_buf_ops	*buf_ops;
>  
>  #if defined(DEBUG) || defined(XFS_WARN)
> @@ -249,6 +273,7 @@ typedef struct xfs_btree_cur
>  #define XFS_BTREE_ROOT_IN_INODE		(1<<1)	/* root may be variable size */
>  #define XFS_BTREE_LASTREC_UPDATE	(1<<2)	/* track last rec externally */
>  #define XFS_BTREE_CRC_BLOCKS		(1<<3)	/* uses extended btree blocks */
> +#define XFS_BTREE_OVERLAPPING		(1<<4)	/* overlapping intervals */
>  
>  
>  #define	XFS_BTREE_NOERROR	0
> @@ -493,5 +518,10 @@ void xfs_btree_get_leaf_keys(struct xfs_btree_cur *cur,
>  void xfs_btree_get_node_keys(struct xfs_btree_cur *cur,
>  		struct xfs_btree_block *block, union xfs_btree_key *key);
>  int xfs_btree_update_keys(struct xfs_btree_cur *cur, int level);
> +void xfs_btree_get_leaf_keys_overlapped(struct xfs_btree_cur *cur,
> +		struct xfs_btree_block *block, union xfs_btree_key *key);
> +void xfs_btree_get_node_keys_overlapped(struct xfs_btree_cur *cur,
> +		struct xfs_btree_block *block, union xfs_btree_key *key);
> +int xfs_btree_update_keys_overlapped(struct xfs_btree_cur *cur, int level);
>  
>  #endif	/* __XFS_BTREE_H__ */
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 1451690..8fb59e6 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -38,6 +38,7 @@ struct xlog_recover_item;
>  struct xfs_buf_log_format;
>  struct xfs_inode_log_format;
>  struct xfs_bmbt_irec;
> +struct xfs_btree_cur;
>  
>  DECLARE_EVENT_CLASS(xfs_attr_list_class,
>  	TP_PROTO(struct xfs_attr_list_context *ctx),
> @@ -2185,6 +2186,41 @@ DEFINE_DISCARD_EVENT(xfs_discard_toosmall);
>  DEFINE_DISCARD_EVENT(xfs_discard_exclude);
>  DEFINE_DISCARD_EVENT(xfs_discard_busy);
>  
> +/* btree cursor events */
> +DECLARE_EVENT_CLASS(xfs_btree_cur_class,
> +	TP_PROTO(struct xfs_btree_cur *cur, int level, struct xfs_buf *bp),
> +	TP_ARGS(cur, level, bp),
> +	TP_STRUCT__entry(
> +		__field(dev_t, dev)
> +		__field(xfs_btnum_t, btnum)
> +		__field(int, level)
> +		__field(int, nlevels)
> +		__field(int, ptr)
> +		__field(xfs_daddr_t, daddr)
> +	),
> +	TP_fast_assign(
> +		__entry->dev = cur->bc_mp->m_super->s_dev;
> +		__entry->btnum = cur->bc_btnum;
> +		__entry->level = level;
> +		__entry->nlevels = cur->bc_nlevels;
> +		__entry->ptr = cur->bc_ptrs[level];
> +		__entry->daddr = bp ? bp->b_bn : -1;
> +	),
> +	TP_printk("dev %d:%d btnum %d level %d/%d ptr %d daddr 0x%llx",
> +		  MAJOR(__entry->dev), MINOR(__entry->dev),
> +		  __entry->btnum,
> +		  __entry->level,
> +		  __entry->nlevels,
> +		  __entry->ptr,
> +		  (unsigned long long)__entry->daddr)
> +)
> +
> +#define DEFINE_BTREE_CUR_EVENT(name) \
> +DEFINE_EVENT(xfs_btree_cur_class, name, \
> +	TP_PROTO(struct xfs_btree_cur *cur, int level, struct xfs_buf *bp), \
> +	TP_ARGS(cur, level, bp))
> +DEFINE_BTREE_CUR_EVENT(xfs_btree_updkeys);
> +
>  #endif /* _TRACE_XFS_H */
>  
>  #undef TRACE_INCLUDE_PATH
> 
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