Conserve stack in dsl_scan_visit()

The dsl_scan_visit() function is a little heavy weight taking 464
bytes on the stack.  This can be easily reduced for little cost by
moving zap_cursor_t and zap_attribute_t off the stack and on to the
heap.  After this change dsl_scan_visit() has been reduced in size
by 320 bytes.

This change was made to reduce stack usage in the dsl_scan_sync()
callpath which is recursive and has been observed to overflow the
stack.

Issue #174

diff --git a/module/zfs/dsl_scan.c b/module/zfs/dsl_scan.c
index e3d2492..ff36ea9 100644 (file)
--- a/module/zfs/dsl_scan.c
+++ b/module/zfs/dsl_scan.c
@@ -1328,8 +1328,8 @@ static void
 dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
 {
        dsl_pool_t *dp = scn->scn_dp;
-       zap_cursor_t zc;
-       zap_attribute_t za;
+       zap_cursor_t *zc;
+       zap_attribute_t *za;
 
        if (scn->scn_phys.scn_ddt_bookmark.ddb_class <=
            scn->scn_phys.scn_ddt_class_max) {
@@ -1377,24 +1377,26 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
         * bookmark so we don't think that we're still trying to resume.
         */
        bzero(&scn->scn_phys.scn_bookmark, sizeof (zbookmark_t));
+       zc = kmem_alloc(sizeof(zap_cursor_t), KM_SLEEP);
+       za = kmem_alloc(sizeof(zap_attribute_t), KM_SLEEP);
 
        /* keep pulling things out of the zap-object-as-queue */
-       while (zap_cursor_init(&zc, dp->dp_meta_objset,
+       while (zap_cursor_init(zc, dp->dp_meta_objset,
            scn->scn_phys.scn_queue_obj),
-           zap_cursor_retrieve(&zc, &za) == 0) {
+           zap_cursor_retrieve(zc, za) == 0) {
                dsl_dataset_t *ds;
                uint64_t dsobj;
 
-               dsobj = strtonum(za.za_name, NULL);
+               dsobj = strtonum(za->za_name, NULL);
                VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
                    scn->scn_phys.scn_queue_obj, dsobj, tx));
 
                /* Set up min/max txg */
                VERIFY3U(0, ==, dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds));
-               if (za.za_first_integer != 0) {
+               if (za->za_first_integer != 0) {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
-                           za.za_first_integer);
+                           za->za_first_integer);
                } else {
                        scn->scn_phys.scn_cur_min_txg =
                            MAX(scn->scn_phys.scn_min_txg,
@@ -1404,11 +1406,14 @@ dsl_scan_visit(dsl_scan_t *scn, dmu_tx_t *tx)
                dsl_dataset_rele(ds, FTAG);
 
                dsl_scan_visitds(scn, dsobj, tx);
-               zap_cursor_fini(&zc);
+               zap_cursor_fini(zc);
                if (scn->scn_pausing)
-                       return;
+                       goto out;
        }
-       zap_cursor_fini(&zc);
+       zap_cursor_fini(zc);
+out:
+       kmem_free(za, sizeof(zap_attribute_t));
+       kmem_free(zc, sizeof(zap_cursor_t));
 }
 
 static int