Tinker with slop space accounting with dedup

* Tinker with slop space accounting with dedup

Do not include the deduplicated space usage in the slop space
reservation, it leads to surprising outcomes.

* Update spa_dedup_dspace sometimes

Sometimes, we get into spa_get_slop_space() with
spa_dedup_dspace=~0ULL, AKA "unset", while spa_dspace is correctly set.

So call the code to update it before we use it if we hit that case.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12271

module/zfs/ddt.c

@@ -503,7 +503,7 @@ ddt_get_dedup_histogram(spa_t *spa, ddt_histogram_t *ddh)
 {
 	for (enum zio_checksum c = 0; c < ZIO_CHECKSUM_FUNCTIONS; c++) {
 		ddt_t *ddt = spa->spa_ddt[c];
-		for (enum ddt_type type = 0; type < DDT_TYPES; type++) {
+		for (enum ddt_type type = 0; type < DDT_TYPES && ddt; type++) {
 			for (enum ddt_class class = 0; class < DDT_CLASSES;
 			    class++) {
 				ddt_histogram_add(ddh,

module/zfs/spa_misc.c

@@ -1789,8 +1789,22 @@ spa_get_worst_case_asize(spa_t *spa, uint64_t lsize)
 uint64_t
 spa_get_slop_space(spa_t *spa)
 {
-	uint64_t space = spa_get_dspace(spa);
-	uint64_t slop = MIN(space >> spa_slop_shift, spa_max_slop);
+	uint64_t space = 0;
+	uint64_t slop = 0;
+
+	/*
+	 * Make sure spa_dedup_dspace has been set.
+	 */
+	if (spa->spa_dedup_dspace == ~0ULL)
+		spa_update_dspace(spa);
+
+	/*
+	 * spa_get_dspace() includes the space only logically "used" by
+	 * deduplicated data, so since it's not useful to reserve more
+	 * space with more deduplicated data, we subtract that out here.
+	 */
+	space = spa_get_dspace(spa) - spa->spa_dedup_dspace;
+	slop = MIN(space >> spa_slop_shift, spa_max_slop);
 
 	/*
 	 * Slop space should be at least spa_min_slop, but no more than half