4047 panic from dbuf_free_range() from dmu_free_object() while doing zfs receive
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>

@@ -38,10 +38,16 @@
 #include <sys/zio.h>
 #include <sys/dmu_zfetch.h>
 #include <sys/sa.h>
 #include <sys/sa_impl.h>
 
+/*
+ * Number of times that zfs_free_range() took the slow path while doing
+ * a zfs receive.  A nonzero value indicates a potential performance problem.
+ */
+uint64_t zfs_free_range_recv_miss;
+
 static void dbuf_destroy(dmu_buf_impl_t *db);
 static boolean_t dbuf_undirty(dmu_buf_impl_t *db, dmu_tx_t *tx);
 static void dbuf_write(dbuf_dirty_record_t *dr, arc_buf_t *data, dmu_tx_t *tx);
 
 /*

@@ -817,24 +823,26 @@
                 end = dn->dn_maxblkid;
                 last_l1 = end >> epbs;
         }
         dprintf_dnode(dn, "start=%llu end=%llu\n", start, end);
 
-        if (dmu_objset_is_receiving(dn->dn_objset)) {
+        mutex_enter(&dn->dn_dbufs_mtx);
+        if (start >= dn->dn_unlisted_l0_blkid * dn->dn_datablksz) {
+                /* There can't be any dbufs in this range; no need to search. */
+                mutex_exit(&dn->dn_dbufs_mtx);
+                return;
+        } else if (dmu_objset_is_receiving(dn->dn_objset)) {
                 /*
-                 * When processing a free record from a zfs receive,
-                 * there should have been no previous modifications to the
-                 * data in this range.  Therefore there should be no dbufs
-                 * in the range.  Searching dn_dbufs for these non-existent
-                 * dbufs can be very expensive, so simply ignore this.
+                 * If we are receiving, we expect there to be no dbufs in
+                 * the range to be freed, because receive modifies each
+                 * block at most once, and in offset order.  If this is
+                 * not the case, it can lead to performance problems,
+                 * so note that we unexpectedly took the slow path.
                  */
-                VERIFY3P(dbuf_find(dn, 0, start), ==, NULL);
-                VERIFY3P(dbuf_find(dn, 0, end), ==, NULL);
-                return;
+                atomic_inc_64(&zfs_free_range_recv_miss);
         }
 
-        mutex_enter(&dn->dn_dbufs_mtx);
         for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
                 db_next = list_next(&dn->dn_dbufs, db);
                 ASSERT(db->db_blkid != DMU_BONUS_BLKID);
 
                 if (db->db_level == 1 &&

@@ -1718,10 +1726,13 @@
                 kmem_cache_free(dbuf_cache, db);
                 mutex_exit(&dn->dn_dbufs_mtx);
                 return (odb);
         }
         list_insert_head(&dn->dn_dbufs, db);
+        if (db->db_level == 0 && db->db_blkid >=
+            dn->dn_unlisted_l0_blkid)
+                dn->dn_unlisted_l0_blkid = db->db_blkid + 1;
         db->db_state = DB_UNCACHED;
         mutex_exit(&dn->dn_dbufs_mtx);
         arc_space_consume(sizeof (dmu_buf_impl_t), ARC_SPACE_OTHER);
 
         if (parent && parent != dn->dn_dbuf)