quota: extend quota for dataset

This patch relax the quota limitation for dataset by around 3%.
What this means is that user can write more data then the quota is
set to. However thanks to that we can get more stable bandwidth, in
case when we are overwriting data in-place, and not consuming any
additional space.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Allan Jude <[email protected]>
Signed-off-by: Mariusz Zaborski <[email protected]>
Sponsored-by: Zededa Inc.
Sponsored-by: Klara Inc.
Closes #13839

module/zfs/dsl_dir.c

@@ -1262,6 +1262,7 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 	uint64_t quota;
 	struct tempreserve *tr;
 	int retval;
+	uint64_t ext_quota;
 	uint64_t ref_rsrv;
 
 top_of_function:
@@ -1337,7 +1338,16 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 	 * on-disk is over quota and there are no pending changes
 	 * or deferred frees (which may free up space for us).
 	 */
-	if (used_on_disk + est_inflight >= quota) {
+	ext_quota = quota >> 5;
+	if (quota == UINT64_MAX)
+		ext_quota = 0;
+
+	if (used_on_disk >= quota) {
+		/* Quota exceeded */
+		mutex_exit(&dd->dd_lock);
+		DMU_TX_STAT_BUMP(dmu_tx_quota);
+		return (retval);
+	} else if (used_on_disk + est_inflight >= quota + ext_quota) {
 		if (est_inflight > 0 || used_on_disk < quota) {
 			retval = SET_ERROR(ERESTART);
 		} else {