Fix race in dnode_check_slots_free()

Currently, dnode_check_slots_free() works by checking dn->dn_type
in the dnode to determine if the dnode is reclaimable. However,
there is a small window of time between dnode_free_sync() in the
first call to dsl_dataset_sync() and when the useraccounting code
is run when the type is set DMU_OT_NONE, but the dnode is not yet
evictable, leading to crashes. This patch adds the ability for
dnodes to track which txg they were last dirtied in and adds a
check for this before performing the reclaim.

This patch also corrects several instances when dn_dirty_link was
treated as a list_node_t when it is technically a multilist_node_t.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tom Caputi <[email protected]>
Closes #7147 
Closes #7388 

1 files changed, 19 insertions, 10 deletions

diff --git a/module/zfs/dnode.c b/module/zfs/dnode.c
index 596983b47..a379527a0 100644
--- a/module/zfs/dnode.c
+++ b/module/zfs/dnode.c
@@ -139,7 +139,7 @@ dnode_cons(void *arg, void *unused, int kmflag)
 	bzero(&dn->dn_next_maxblkid[0], sizeof (dn->dn_next_maxblkid));
 
 	for (i = 0; i < TXG_SIZE; i++) {
-		list_link_init(&dn->dn_dirty_link[i]);
+		multilist_link_init(&dn->dn_dirty_link[i]);
 		dn->dn_free_ranges[i] = NULL;
 		list_create(&dn->dn_dirty_records[i],
 		    sizeof (dbuf_dirty_record_t),
@@ -149,6 +149,7 @@ dnode_cons(void *arg, void *unused, int kmflag)
 	dn->dn_allocated_txg = 0;
 	dn->dn_free_txg = 0;
 	dn->dn_assigned_txg = 0;
+	dn->dn_dirty_txg = 0;
 	dn->dn_dirtyctx = 0;
 	dn->dn_dirtyctx_firstset = NULL;
 	dn->dn_bonus = NULL;
@@ -188,7 +189,7 @@ dnode_dest(void *arg, void *unused)
 	ASSERT(!list_link_active(&dn->dn_link));
 
 	for (i = 0; i < TXG_SIZE; i++) {
-		ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
+		ASSERT(!multilist_link_active(&dn->dn_dirty_link[i]));
 		ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
 		list_destroy(&dn->dn_dirty_records[i]);
 		ASSERT0(dn->dn_next_nblkptr[i]);
@@ -204,6 +205,7 @@ dnode_dest(void *arg, void *unused)
 	ASSERT0(dn->dn_allocated_txg);
 	ASSERT0(dn->dn_free_txg);
 	ASSERT0(dn->dn_assigned_txg);
+	ASSERT0(dn->dn_dirty_txg);
 	ASSERT0(dn->dn_dirtyctx);
 	ASSERT3P(dn->dn_dirtyctx_firstset, ==, NULL);
 	ASSERT3P(dn->dn_bonus, ==, NULL);
@@ -530,6 +532,7 @@ dnode_destroy(dnode_t *dn)
 	dn->dn_allocated_txg = 0;
 	dn->dn_free_txg = 0;
 	dn->dn_assigned_txg = 0;
+	dn->dn_dirty_txg = 0;
 
 	dn->dn_dirtyctx = 0;
 	if (dn->dn_dirtyctx_firstset != NULL) {
@@ -601,6 +604,7 @@ dnode_allocate(dnode_t *dn, dmu_object_type_t ot, int blocksize, int ibs,
 	ASSERT0(dn->dn_maxblkid);
 	ASSERT0(dn->dn_allocated_txg);
 	ASSERT0(dn->dn_assigned_txg);
+	ASSERT0(dn->dn_dirty_txg);
 	ASSERT(refcount_is_zero(&dn->dn_tx_holds));
 	ASSERT3U(refcount_count(&dn->dn_holds), <=, 1);
 	ASSERT(avl_is_empty(&dn->dn_dbufs));
@@ -614,7 +618,7 @@ dnode_allocate(dnode_t *dn, dmu_object_type_t ot, int blocksize, int ibs,
 		ASSERT0(dn->dn_rm_spillblk[i]);
 		ASSERT0(dn->dn_next_blksz[i]);
 		ASSERT0(dn->dn_next_maxblkid[i]);
-		ASSERT(!list_link_active(&dn->dn_dirty_link[i]));
+		ASSERT(!multilist_link_active(&dn->dn_dirty_link[i]));
 		ASSERT3P(list_head(&dn->dn_dirty_records[i]), ==, NULL);
 		ASSERT3P(dn->dn_free_ranges[i], ==, NULL);
 	}
@@ -792,6 +796,7 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	ndn->dn_allocated_txg = odn->dn_allocated_txg;
 	ndn->dn_free_txg = odn->dn_free_txg;
 	ndn->dn_assigned_txg = odn->dn_assigned_txg;
+	ndn->dn_dirty_txg = odn->dn_dirty_txg;
 	ndn->dn_dirtyctx = odn->dn_dirtyctx;
 	ndn->dn_dirtyctx_firstset = odn->dn_dirtyctx_firstset;
 	ASSERT(refcount_count(&odn->dn_tx_holds) == 0);
@@ -860,6 +865,7 @@ dnode_move_impl(dnode_t *odn, dnode_t *ndn)
 	odn->dn_allocated_txg = 0;
 	odn->dn_free_txg = 0;
 	odn->dn_assigned_txg = 0;
+	odn->dn_dirty_txg = 0;
 	odn->dn_dirtyctx = 0;
 	odn->dn_dirtyctx_firstset = NULL;
 	odn->dn_have_spill = B_FALSE;
@@ -1086,6 +1092,10 @@ dnode_check_slots_free(dnode_children_t *children, int idx, int slots)
 {
 	ASSERT3S(idx + slots, <=, DNODES_PER_BLOCK);
 
+	/*
+	 * If all dnode slots are either already free or
+	 * evictable return B_TRUE.
+	 */
 	for (int i = idx; i < idx + slots; i++) {
 		dnode_handle_t *dnh = &children->dnc_children[i];
 		dnode_t *dn = dnh->dnh_dnode;
@@ -1094,18 +1104,17 @@ dnode_check_slots_free(dnode_children_t *children, int idx, int slots)
 			continue;
 		} else if (DN_SLOT_IS_PTR(dn)) {
 			mutex_enter(&dn->dn_mtx);
-			dmu_object_type_t type = dn->dn_type;
+			boolean_t can_free = (dn->dn_type == DMU_OT_NONE &&
+			    !DNODE_IS_DIRTY(dn));
 			mutex_exit(&dn->dn_mtx);
 
-			if (type != DMU_OT_NONE)
+			if (!can_free)
 				return (B_FALSE);
-
-			continue;
+			else
+				continue;
 		} else {
 			return (B_FALSE);
 		}
-
-		return (B_FALSE);
 	}
 
 	return (B_TRUE);
@@ -1633,7 +1642,7 @@ dnode_setdirty(dnode_t *dn, dmu_tx_t *tx)
 	/*
 	 * If we are already marked dirty, we're done.
 	 */
-	if (list_link_active(&dn->dn_dirty_link[txg & TXG_MASK])) {
+	if (multilist_link_active(&dn->dn_dirty_link[txg & TXG_MASK])) {
 		multilist_sublist_unlock(mls);
 		return;
 	}