xfs: exact busy extent tracking

Update the extent tree in case we have to reuse a busy extent, so that it
always is kept uptodate.  This is done by replacing the busy list searches
with a new xfs_alloc_busy_reuse helper, which updates the busy extent tree
in case of a reuse.  This allows us to allow reusing metadata extents
unconditionally, and thus avoid log forces especially for allocation btree
blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

1 files changed, 218 insertions, 155 deletions

diff --git a/fs/xfs/xfs_alloc.c b/fs/xfs/xfs_alloc.c
index ff54ef428348..53157d4d5e8b 100644
--- a/fs/xfs/xfs_alloc.c
+++ b/fs/xfs/xfs_alloc.c
@@ -1396,8 +1396,9 @@ xfs_alloc_ag_vextent_small(
 		if (error)
 			goto error0;
 		if (fbno != NULLAGBLOCK) {
-			if (xfs_alloc_busy_search(args->mp, args->agno, fbno, 1))
-				xfs_trans_set_sync(args->tp);
+			xfs_alloc_busy_reuse(args->mp, args->agno, fbno, 1,
+					     args->userdata);
+
 			if (args->userdata) {
 				xfs_buf_t	*bp;
 
@@ -2475,100 +2476,6 @@ error0:
 	return error;
 }
 
-
-/*
- * AG Busy list management
- * The busy list contains block ranges that have been freed but whose
- * transactions have not yet hit disk.  If any block listed in a busy
- * list is reused, the transaction that freed it must be forced to disk
- * before continuing to use the block.
- *
- * xfs_alloc_busy_insert - add to the per-ag busy list
- * xfs_alloc_busy_clear - remove an item from the per-ag busy list
- * xfs_alloc_busy_search - search for a busy extent
- */
-
-/*
- * Insert a new extent into the busy tree.
- *
- * The busy extent tree is indexed by the start block of the busy extent.
- * there can be multiple overlapping ranges in the busy extent tree but only
- * ever one entry at a given start block. The reason for this is that
- * multi-block extents can be freed, then smaller chunks of that extent
- * allocated and freed again before the first transaction commit is on disk.
- * If the exact same start block is freed a second time, we have to wait for
- * that busy extent to pass out of the tree before the new extent is inserted.
- * There are two main cases we have to handle here.
- *
- * The first case is a transaction that triggers a "free - allocate - free"
- * cycle. This can occur during btree manipulations as a btree block is freed
- * to the freelist, then allocated from the free list, then freed again. In
- * this case, the second extxpnet free is what triggers the duplicate and as
- * such the transaction IDs should match. Because the extent was allocated in
- * this transaction, the transaction must be marked as synchronous. This is
- * true for all cases where the free/alloc/free occurs in the one transaction,
- * hence the addition of the ASSERT(tp->t_flags & XFS_TRANS_SYNC) to this case.
- * This serves to catch violations of the second case quite effectively.
- *
- * The second case is where the free/alloc/free occur in different
- * transactions. In this case, the thread freeing the extent the second time
- * can't mark the extent busy immediately because it is already tracked in a
- * transaction that may be committing.  When the log commit for the existing
- * busy extent completes, the busy extent will be removed from the tree. If we
- * allow the second busy insert to continue using that busy extent structure,
- * it can be freed before this transaction is safely in the log.  Hence our
- * only option in this case is to force the log to remove the existing busy
- * extent from the list before we insert the new one with the current
- * transaction ID.
- *
- * The problem we are trying to avoid in the free-alloc-free in separate
- * transactions is most easily described with a timeline:
- *
- *      Thread 1	Thread 2	Thread 3	xfslogd
- *	xact alloc
- *	free X
- *	mark busy
- *	commit xact
- *	free xact
- *			xact alloc
- *			alloc X
- *			busy search
- *			mark xact sync
- *			commit xact
- *			free xact
- *			force log
- *			checkpoint starts
- *			....
- *					xact alloc
- *					free X
- *					mark busy
- *					finds match
- *					*** KABOOM! ***
- *					....
- *							log IO completes
- *							unbusy X
- *			checkpoint completes
- *
- * By issuing a log force in thread 3 @ "KABOOM", the thread will block until
- * the checkpoint completes, and the busy extent it matched will have been
- * removed from the tree when it is woken. Hence it can then continue safely.
- *
- * However, to ensure this matching process is robust, we need to use the
- * transaction ID for identifying transaction, as delayed logging results in
- * the busy extent and transaction lifecycles being different. i.e. the busy
- * extent is active for a lot longer than the transaction.  Hence the
- * transaction structure can be freed and reallocated, then mark the same
- * extent busy again in the new transaction. In this case the new transaction
- * will have a different tid but can have the same address, and hence we need
- * to check against the tid.
- *
- * Future: for delayed logging, we could avoid the log force if the extent was
- * first freed in the current checkpoint sequence. This, however, requires the
- * ability to pin the current checkpoint in memory until this transaction
- * commits to ensure that both the original free and the current one combine
- * logically into the one checkpoint. If the checkpoint sequences are
- * different, however, we still need to wait on a log force.
- */
 void
 xfs_alloc_busy_insert(
 	struct xfs_trans	*tp,
@@ -2580,9 +2487,7 @@ xfs_alloc_busy_insert(
 	struct xfs_busy_extent	*busyp;
 	struct xfs_perag	*pag;
 	struct rb_node		**rbp;
-	struct rb_node		*parent;
-	int			match;
-
+	struct rb_node		*parent = NULL;
 
 	new = kmem_zalloc(sizeof(struct xfs_busy_extent), KM_MAYFAIL);
 	if (!new) {
@@ -2591,7 +2496,7 @@ xfs_alloc_busy_insert(
 		 * block, make this a synchronous transaction to insure that
 		 * the block is not reused before this transaction commits.
 		 */
-		trace_xfs_alloc_busy(tp, agno, bno, len, 1);
+		trace_xfs_alloc_busy_enomem(tp->t_mountp, agno, bno, len);
 		xfs_trans_set_sync(tp);
 		return;
 	}
@@ -2599,66 +2504,28 @@ xfs_alloc_busy_insert(
 	new->agno = agno;
 	new->bno = bno;
 	new->length = len;
-	new->tid = xfs_log_get_trans_ident(tp);
-
 	INIT_LIST_HEAD(&new->list);
 
 	/* trace before insert to be able to see failed inserts */
-	trace_xfs_alloc_busy(tp, agno, bno, len, 0);
+	trace_xfs_alloc_busy(tp->t_mountp, agno, bno, len);
 
 	pag = xfs_perag_get(tp->t_mountp, new->agno);
-restart:
 	spin_lock(&pag->pagb_lock);
 	rbp = &pag->pagb_tree.rb_node;
-	parent = NULL;
-	busyp = NULL;
-	match = 0;
-	while (*rbp && match >= 0) {
+	while (*rbp) {
 		parent = *rbp;
 		busyp = rb_entry(parent, struct xfs_busy_extent, rb_node);
 
 		if (new->bno < busyp->bno) {
-			/* may overlap, but exact start block is lower */
 			rbp = &(*rbp)->rb_left;
-			if (new->bno + new->length > busyp->bno)
-				match = busyp->tid == new->tid ? 1 : -1;
+			ASSERT(new->bno + new->length <= busyp->bno);
 		} else if (new->bno > busyp->bno) {
-			/* may overlap, but exact start block is higher */
 			rbp = &(*rbp)->rb_right;
-			if (bno < busyp->bno + busyp->length)
-				match = busyp->tid == new->tid ? 1 : -1;
+			ASSERT(bno >= busyp->bno + busyp->length);
 		} else {
-			match = busyp->tid == new->tid ? 1 : -1;
-			break;
+			ASSERT(0);
 		}
 	}
-	if (match < 0) {
-		/* overlap marked busy in different transaction */
-		spin_unlock(&pag->pagb_lock);
-		xfs_log_force(tp->t_mountp, XFS_LOG_SYNC);
-		goto restart;
-	}
-	if (match > 0) {
-		/*
-		 * overlap marked busy in same transaction. Update if exact
-		 * start block match, otherwise combine the busy extents into
-		 * a single range.
-		 */
-		if (busyp->bno == new->bno) {
-			busyp->length = max(busyp->length, new->length);
-			spin_unlock(&pag->pagb_lock);
-			ASSERT(tp->t_flags & XFS_TRANS_SYNC);
-			xfs_perag_put(pag);
-			kmem_free(new);
-			return;
-		}
-		rb_erase(&busyp->rb_node, &pag->pagb_tree);
-		new->length = max(busyp->bno + busyp->length,
-					new->bno + new->length) -
-				min(busyp->bno, new->bno);
-		new->bno = min(busyp->bno, new->bno);
-	} else
-		busyp = NULL;
 
 	rb_link_node(&new->rb_node, parent, rbp);
 	rb_insert_color(&new->rb_node, &pag->pagb_tree);
@@ -2666,7 +2533,6 @@ restart:
 	list_add(&new->list, &tp->t_busy);
 	spin_unlock(&pag->pagb_lock);
 	xfs_perag_put(pag);
-	kmem_free(busyp);
 }
 
 /*
@@ -2715,12 +2581,196 @@ xfs_alloc_busy_search(
 		}
 	}
 	spin_unlock(&pag->pagb_lock);
-	trace_xfs_alloc_busysearch(mp, agno, bno, len, !!match);
 	xfs_perag_put(pag);
 	return match;
 }
 
 /*
+ * The found free extent [fbno, fend] overlaps part or all of the given busy
+ * extent.  If the overlap covers the beginning, the end, or all of the busy
+ * extent, the overlapping portion can be made unbusy and used for the
+ * allocation.  We can't split a busy extent because we can't modify a
+ * transaction/CIL context busy list, but we can update an entries block
+ * number or length.
+ *
+ * Returns true if the extent can safely be reused, or false if the search
+ * needs to be restarted.
+ */
+STATIC bool
+xfs_alloc_busy_update_extent(
+	struct xfs_mount	*mp,
+	struct xfs_perag	*pag,
+	struct xfs_busy_extent	*busyp,
+	xfs_agblock_t		fbno,
+	xfs_extlen_t		flen,
+	bool			userdata)
+{
+	xfs_agblock_t		fend = fbno + flen;
+	xfs_agblock_t		bbno = busyp->bno;
+	xfs_agblock_t		bend = bbno + busyp->length;
+
+	/*
+	 * If there is a busy extent overlapping a user allocation, we have
+	 * no choice but to force the log and retry the search.
+	 *
+	 * Fortunately this does not happen during normal operation, but
+	 * only if the filesystem is very low on space and has to dip into
+	 * the AGFL for normal allocations.
+	 */
+	if (userdata)
+		goto out_force_log;
+
+	if (bbno < fbno && bend > fend) {
+		/*
+		 * Case 1:
+		 *    bbno           bend
+		 *    +BBBBBBBBBBBBBBBBB+
+		 *        +---------+
+		 *        fbno   fend
+		 */
+
+		/*
+		 * We would have to split the busy extent to be able to track
+		 * it correct, which we cannot do because we would have to
+		 * modify the list of busy extents attached to the transaction
+		 * or CIL context, which is immutable.
+		 *
+		 * Force out the log to clear the busy extent and retry the
+		 * search.
+		 */
+		goto out_force_log;
+	} else if (bbno >= fbno && bend <= fend) {
+		/*
+		 * Case 2:
+		 *    bbno           bend
+		 *    +BBBBBBBBBBBBBBBBB+
+		 *    +-----------------+
+		 *    fbno           fend
+		 *
+		 * Case 3:
+		 *    bbno           bend
+		 *    +BBBBBBBBBBBBBBBBB+
+		 *    +--------------------------+
+		 *    fbno                    fend
+		 *
+		 * Case 4:
+		 *             bbno           bend
+		 *             +BBBBBBBBBBBBBBBBB+
+		 *    +--------------------------+
+		 *    fbno                    fend
+		 *
+		 * Case 5:
+		 *             bbno           bend
+		 *             +BBBBBBBBBBBBBBBBB+
+		 *    +-----------------------------------+
+		 *    fbno                             fend
+		 *
+		 */
+
+		/*
+		 * The busy extent is fully covered by the extent we are
+		 * allocating, and can simply be removed from the rbtree.
+		 * However we cannot remove it from the immutable list
+		 * tracking busy extents in the transaction or CIL context,
+		 * so set the length to zero to mark it invalid.
+		 *
+		 * We also need to restart the busy extent search from the
+		 * tree root, because erasing the node can rearrange the
+		 * tree topology.
+		 */
+		rb_erase(&busyp->rb_node, &pag->pagb_tree);
+		busyp->length = 0;
+		return false;
+	} else if (fend < bend) {
+		/*
+		 * Case 6:
+		 *              bbno           bend
+		 *             +BBBBBBBBBBBBBBBBB+
+		 *             +---------+
+		 *             fbno   fend
+		 *
+		 * Case 7:
+		 *             bbno           bend
+		 *             +BBBBBBBBBBBBBBBBB+
+		 *    +------------------+
+		 *    fbno            fend
+		 *
+		 */
+		busyp->bno = fend;
+	} else if (bbno < fbno) {
+		/*
+		 * Case 8:
+		 *    bbno           bend
+		 *    +BBBBBBBBBBBBBBBBB+
+		 *        +-------------+
+		 *        fbno       fend
+		 *
+		 * Case 9:
+		 *    bbno           bend
+		 *    +BBBBBBBBBBBBBBBBB+
+		 *        +----------------------+
+		 *        fbno                fend
+		 */
+		busyp->length = fbno - busyp->bno;
+	} else {
+		ASSERT(0);
+	}
+
+	trace_xfs_alloc_busy_reuse(mp, pag->pag_agno, fbno, flen);
+	return true;
+
+out_force_log:
+	spin_unlock(&pag->pagb_lock);
+	xfs_log_force(mp, XFS_LOG_SYNC);
+	trace_xfs_alloc_busy_force(mp, pag->pag_agno, fbno, flen);
+	spin_lock(&pag->pagb_lock);
+	return false;
+}
+
+
+/*
+ * For a given extent [fbno, flen], make sure we can reuse it safely.
+ */
+void
+xfs_alloc_busy_reuse(
+	struct xfs_mount	*mp,
+	xfs_agnumber_t		agno,
+	xfs_agblock_t		fbno,
+	xfs_extlen_t		flen,
+	bool			userdata)
+{
+	struct xfs_perag	*pag;
+	struct rb_node		*rbp;
+
+	ASSERT(flen > 0);
+
+	pag = xfs_perag_get(mp, agno);
+	spin_lock(&pag->pagb_lock);
+restart:
+	rbp = pag->pagb_tree.rb_node;
+	while (rbp) {
+		struct xfs_busy_extent *busyp =
+			rb_entry(rbp, struct xfs_busy_extent, rb_node);
+		xfs_agblock_t	bbno = busyp->bno;
+		xfs_agblock_t	bend = bbno + busyp->length;
+
+		if (fbno + flen <= bbno) {
+			rbp = rbp->rb_left;
+			continue;
+		} else if (fbno >= bend) {
+			rbp = rbp->rb_right;
+			continue;
+		}
+
+		if (!xfs_alloc_busy_update_extent(mp, pag, busyp, fbno, flen,
+						  userdata))
+			goto restart;
+	}
+	spin_unlock(&pag->pagb_lock);
+	xfs_perag_put(pag);
+}
+
+/*
  * For a given extent [fbno, flen], search the busy extent list to find a
  * subset of the extent that is not busy.  If *rlen is smaller than
  * args->minlen no suitable extent could be found, and the higher level
@@ -2734,13 +2784,16 @@ xfs_alloc_busy_trim(
 	xfs_agblock_t		*rbno,
 	xfs_extlen_t		*rlen)
 {
-	xfs_agblock_t		fbno = bno;
-	xfs_extlen_t		flen = len;
+	xfs_agblock_t		fbno;
+	xfs_extlen_t		flen;
 	struct rb_node		*rbp;
 
-	ASSERT(flen > 0);
+	ASSERT(len > 0);
 
 	spin_lock(&args->pag->pagb_lock);
+restart:
+	fbno = bno;
+	flen = len;
 	rbp = args->pag->pagb_tree.rb_node;
 	while (rbp && flen >= args->minlen) {
 		struct xfs_busy_extent *busyp =
@@ -2757,6 +2810,18 @@ xfs_alloc_busy_trim(
 			continue;
 		}
 
+		/*
+		 * If this is a metadata allocation, try to reuse the busy
+		 * extent instead of trimming the allocation.
+		 */
+		if (!args->userdata) {
+			if (!xfs_alloc_busy_update_extent(args->mp, args->pag,
+							  busyp, fbno, flen,
+							  false))
+				goto restart;
+			continue;
+		}
+
 		if (bbno <= fbno) {
 			/* start overlap */
 
@@ -2906,17 +2971,15 @@ xfs_alloc_busy_clear(
 {
 	struct xfs_perag	*pag;
 
-	trace_xfs_alloc_unbusy(mp, busyp->agno, busyp->bno,
-						busyp->length);
-
-	ASSERT(xfs_alloc_busy_search(mp, busyp->agno, busyp->bno,
-						busyp->length) == 1);
-
 	list_del_init(&busyp->list);
 
 	pag = xfs_perag_get(mp, busyp->agno);
 	spin_lock(&pag->pagb_lock);
-	rb_erase(&busyp->rb_node, &pag->pagb_tree);
+	if (busyp->length) {
+		trace_xfs_alloc_busy_clear(mp, busyp->agno, busyp->bno,
+						busyp->length);
+		rb_erase(&busyp->rb_node, &pag->pagb_tree);
+	}
 	spin_unlock(&pag->pagb_lock);
 	xfs_perag_put(pag);