4631 zvol_get_stats triggering too many reads

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

usr/src/uts/common/fs/zfs/arc.c

@@ -105,7 +105,7 @@
  * with the buffer may be evicted prior to the callback.  The callback
  * must be made with *no locks held* (to prevent deadlock).  Additionally,
  * the users of callbacks must ensure that their private data is
- * protected from simultaneous callbacks from arc_buf_evict()
+ * protected from simultaneous callbacks from arc_clear_callback()
  * and arc_do_user_evicts().
  *
  * Note that the majority of the performance stats are manipulated
@@ -1495,8 +1495,12 @@ arc_buf_data_free(arc_buf_t *buf, void (*free_func)(void *, size_t))
 	}
 }
 
+/*
+ * Free up buf->b_data and if 'remove' is set, then pull the
+ * arc_buf_t off of the the arc_buf_hdr_t's list and free it.
+ */
 static void
-arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t all)
+arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t remove)
 {
 	arc_buf_t **bufp;
 
@@ -1547,7 +1551,7 @@ arc_buf_destroy(arc_buf_t *buf, boolean_t recycle, boolean_t all)
 	}
 
 	/* only remove the buf if requested */
-	if (!all)
+	if (!remove)
 		return;
 
 	/* remove the buf from the hdr list */
@@ -2121,7 +2125,7 @@ arc_do_user_evicts(void)
 		mutex_exit(&arc_eviction_mtx);
 
 		if (buf->b_efunc != NULL)
-			VERIFY(buf->b_efunc(buf) == 0);
+			VERIFY0(buf->b_efunc(buf->b_private));
 
 		buf->b_efunc = NULL;
 		buf->b_private = NULL;
@@ -3240,16 +3244,25 @@ arc_freed(spa_t *spa, const blkptr_t *bp)
 }
 
 /*
- * This is used by the DMU to let the ARC know that a buffer is
- * being evicted, so the ARC should clean up.  If this arc buf
- * is not yet in the evicted state, it will be put there.
+ * Clear the user eviction callback set by arc_set_callback(), first calling
+ * it if it exists.  Because the presence of a callback keeps an arc_buf cached
+ * clearing the callback may result in the arc_buf being destroyed.  However,
+ * it will not result in the *last* arc_buf being destroyed, hence the data
+ * will remain cached in the ARC. We make a copy of the arc buffer here so
+ * that we can process the callback without holding any locks.
+ *
+ * It's possible that the callback is already in the process of being cleared
+ * by another thread.  In this case we can not clear the callback.
+ *
+ * Returns B_TRUE if the callback was successfully called and cleared.
  */
-int
-arc_buf_evict(arc_buf_t *buf)
+boolean_t
+arc_clear_callback(arc_buf_t *buf)
 {
 	arc_buf_hdr_t *hdr;
 	kmutex_t *hash_lock;
-	arc_buf_t **bufp;
+	arc_evict_func_t *efunc = buf->b_efunc;
+	void *private = buf->b_private;
 
 	mutex_enter(&buf->b_evict_lock);
 	hdr = buf->b_hdr;
@@ -3259,17 +3272,16 @@ arc_buf_evict(arc_buf_t *buf)
 		 */
 		ASSERT(buf->b_data == NULL);
 		mutex_exit(&buf->b_evict_lock);
-		return (0);
+		return (B_FALSE);
 	} else if (buf->b_data == NULL) {
-		arc_buf_t copy = *buf; /* structure assignment */
 		/*
 		 * We are on the eviction list; process this buffer now
 		 * but let arc_do_user_evicts() do the reaping.
 		 */
 		buf->b_efunc = NULL;
 		mutex_exit(&buf->b_evict_lock);
-		VERIFY(copy.b_efunc(&copy) == 0);
-		return (1);
+		VERIFY0(efunc(private));
+		return (B_TRUE);
 	}
 	hash_lock = HDR_LOCK(hdr);
 	mutex_enter(hash_lock);
@@ -3279,48 +3291,21 @@ arc_buf_evict(arc_buf_t *buf)
 	ASSERT3U(refcount_count(&hdr->b_refcnt), <, hdr->b_datacnt);
 	ASSERT(hdr->b_state == arc_mru || hdr->b_state == arc_mfu);
 
-	/*
-	 * Pull this buffer off of the hdr
-	 */
-	bufp = &hdr->b_buf;
-	while (*bufp != buf)
-		bufp = &(*bufp)->b_next;
-	*bufp = buf->b_next;
-
-	ASSERT(buf->b_data != NULL);
-	arc_buf_destroy(buf, FALSE, FALSE);
-
-	if (hdr->b_datacnt == 0) {
-		arc_state_t *old_state = hdr->b_state;
-		arc_state_t *evicted_state;
-
-		ASSERT(hdr->b_buf == NULL);
-		ASSERT(refcount_is_zero(&hdr->b_refcnt));
-
-		evicted_state =
-		    (old_state == arc_mru) ? arc_mru_ghost : arc_mfu_ghost;
-
-		mutex_enter(&old_state->arcs_mtx);
-		mutex_enter(&evicted_state->arcs_mtx);
-
-		arc_change_state(evicted_state, hdr, hash_lock);
-		ASSERT(HDR_IN_HASH_TABLE(hdr));
-		hdr->b_flags |= ARC_IN_HASH_TABLE;
-		hdr->b_flags &= ~ARC_BUF_AVAILABLE;
+	buf->b_efunc = NULL;
+	buf->b_private = NULL;
 
-		mutex_exit(&evicted_state->arcs_mtx);
-		mutex_exit(&old_state->arcs_mtx);
+	if (hdr->b_datacnt > 1) {
+		mutex_exit(&buf->b_evict_lock);
+		arc_buf_destroy(buf, FALSE, TRUE);
+	} else {
+		ASSERT(buf == hdr->b_buf);
+		hdr->b_flags |= ARC_BUF_AVAILABLE;
+		mutex_exit(&buf->b_evict_lock);
 	}
-	mutex_exit(hash_lock);
-	mutex_exit(&buf->b_evict_lock);
 
-	VERIFY(buf->b_efunc(buf) == 0);
-	buf->b_efunc = NULL;
-	buf->b_private = NULL;
-	buf->b_hdr = NULL;
-	buf->b_next = NULL;
-	kmem_cache_free(buf_cache, buf);
-	return (1);
+	mutex_exit(hash_lock);
+	VERIFY0(efunc(private));
+	return (B_TRUE);
 }
 
 /*
@@ -3466,17 +3451,6 @@ arc_released(arc_buf_t *buf)
 	return (released);
 }
 
-int
-arc_has_callback(arc_buf_t *buf)
-{
-	int callback;
-
-	mutex_enter(&buf->b_evict_lock);
-	callback = (buf->b_efunc != NULL);
-	mutex_exit(&buf->b_evict_lock);
-	return (callback);
-}
-
 #ifdef ZFS_DEBUG
 int
 arc_referenced(arc_buf_t *buf)

usr/src/uts/common/fs/zfs/dbuf.c

@@ -181,8 +181,7 @@ dbuf_hash_insert(dmu_buf_impl_t *db)
 }
 
 /*
- * Remove an entry from the hash table.  This operation will
- * fail if there are any existing holds on the db.
+ * Remove an entry from the hash table.  It must be in the EVICTING state.
  */
 static void
 dbuf_hash_remove(dmu_buf_impl_t *db)
@@ -194,7 +193,7 @@ dbuf_hash_remove(dmu_buf_impl_t *db)
 	dmu_buf_impl_t *dbf, **dbp;
 
 	/*
-	 * We musn't hold db_mtx to maintin lock ordering:
+	 * We musn't hold db_mtx to maintain lock ordering:
 	 * DBUF_HASH_MUTEX > db_mtx.
 	 */
 	ASSERT(refcount_is_zero(&db->db_holds));
@@ -431,7 +430,6 @@ static void
 dbuf_set_data(dmu_buf_impl_t *db, arc_buf_t *buf)
 {
 	ASSERT(MUTEX_HELD(&db->db_mtx));
-	ASSERT(db->db_buf == NULL || !arc_has_callback(db->db_buf));
 	db->db_buf = buf;
 	if (buf != NULL) {
 		ASSERT(buf->b_data != NULL);
@@ -1544,20 +1542,23 @@ dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx)
  * when we are not holding the dn_dbufs_mtx, we can't clear the
  * entry in the dn_dbufs list.  We have to wait until dbuf_destroy()
  * in this case.  For callers from the DMU we will usually see:
- *	dbuf_clear()->arc_buf_evict()->dbuf_do_evict()->dbuf_destroy()
+ *	dbuf_clear()->arc_clear_callback()->dbuf_do_evict()->dbuf_destroy()
  * For the arc callback, we will usually see:
  *	dbuf_do_evict()->dbuf_clear();dbuf_destroy()
  * Sometimes, though, we will get a mix of these two:
- *	DMU: dbuf_clear()->arc_buf_evict()
+ *	DMU: dbuf_clear()->arc_clear_callback()
  *	ARC: dbuf_do_evict()->dbuf_destroy()
+ *
+ * This routine will dissociate the dbuf from the arc, by calling
+ * arc_clear_callback(), but will not evict the data from the ARC.
  */
 void
 dbuf_clear(dmu_buf_impl_t *db)
 {
 	dnode_t *dn;
 	dmu_buf_impl_t *parent = db->db_parent;
 	dmu_buf_impl_t *dndb;
-	int dbuf_gone = FALSE;
+	boolean_t dbuf_gone = B_FALSE;
 
 	ASSERT(MUTEX_HELD(&db->db_mtx));
 	ASSERT(refcount_is_zero(&db->db_holds));
@@ -1603,7 +1604,7 @@ dbuf_clear(dmu_buf_impl_t *db)
 	}
 
 	if (db->db_buf)
-		dbuf_gone = arc_buf_evict(db->db_buf);
+		dbuf_gone = arc_clear_callback(db->db_buf);
 
 	if (!dbuf_gone)
 		mutex_exit(&db->db_mtx);
@@ -1771,8 +1772,7 @@ dbuf_create(dnode_t *dn, uint8_t level, uint64_t blkid,
 static int
 dbuf_do_evict(void *private)
 {
-	arc_buf_t *buf = private;
-	dmu_buf_impl_t *db = buf->b_private;
+	dmu_buf_impl_t *db = private;
 
 	if (!MUTEX_HELD(&db->db_mtx))
 		mutex_enter(&db->db_mtx);
@@ -2135,11 +2135,23 @@ dbuf_rele_and_unlock(dmu_buf_impl_t *db, void *tag)
 			 * block on-disk. If so, then we simply evict
 			 * ourselves.
 			 */
-			if (!DBUF_IS_CACHEABLE(db) ||
-			    arc_buf_eviction_needed(db->db_buf))
+			if (!DBUF_IS_CACHEABLE(db)) {
+				if (db->db_blkptr != NULL &&
+				    !BP_IS_HOLE(db->db_blkptr) &&
+				    !BP_IS_EMBEDDED(db->db_blkptr)) {
+					spa_t *spa =
+					    dmu_objset_spa(db->db_objset);
+					blkptr_t bp = *db->db_blkptr;
+					dbuf_clear(db);
+					arc_freed(spa, &bp);
+				} else {
+					dbuf_clear(db);
+				}
+			} else if (arc_buf_eviction_needed(db->db_buf)) {
 				dbuf_clear(db);
-			else
+			} else {
 				mutex_exit(&db->db_mtx);
+			}
 		}
 	} else {
 		mutex_exit(&db->db_mtx);

usr/src/uts/common/fs/zfs/sys/arc.h

@@ -95,7 +95,6 @@ boolean_t arc_buf_remove_ref(arc_buf_t *buf, void *tag);
 int arc_buf_size(arc_buf_t *buf);
 void arc_release(arc_buf_t *buf, void *tag);
 int arc_released(arc_buf_t *buf);
-int arc_has_callback(arc_buf_t *buf);
 void arc_buf_freeze(arc_buf_t *buf);
 void arc_buf_thaw(arc_buf_t *buf);
 boolean_t arc_buf_eviction_needed(arc_buf_t *buf);
@@ -114,7 +113,7 @@ zio_t *arc_write(zio_t *pio, spa_t *spa, uint64_t txg,
 void arc_freed(spa_t *spa, const blkptr_t *bp);
 
 void arc_set_callback(arc_buf_t *buf, arc_evict_func_t *func, void *private);
-int arc_buf_evict(arc_buf_t *buf);
+boolean_t arc_clear_callback(arc_buf_t *buf);
 
 void arc_flush(spa_t *spa);
 void arc_tempreserve_clear(uint64_t reserve);