L2ARC: Restrict write size to 1/4 of the device

PR openzfs#15457 exposed weird logic in L2ARC write sizing. If it appeared
bigger than device size, instead of liming write it reset all the
system-wide tunables to their default.  Aside of being excessive,
it did not actually help with the problem, still allowing infinite
loop to happen.

This patch removes the tunables reverting logic, but instead limits
L2ARC writes (or at least eviction/trim) to 1/4 of the capacity.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.

module/zfs/arc.c

@@ -8035,9 +8035,8 @@ l2arc_write_size(l2arc_dev_t *dev)
 	 */
 	size = l2arc_write_max;
 	if (size == 0) {
-		cmn_err(CE_NOTE, "Bad value for l2arc_write_max, value must "
-		    "be greater than zero, resetting it to the default (%d)",
-		    L2ARC_WRITE_SIZE);
+		cmn_err(CE_NOTE, "l2arc_write_max must be greater than zero, "
+		    "resetting it to the default (%d)", L2ARC_WRITE_SIZE);
 		size = l2arc_write_max = L2ARC_WRITE_SIZE;
 	}
 
@@ -8060,30 +8059,7 @@ l2arc_write_size(l2arc_dev_t *dev)
 	 * device. This is important in l2arc_evict(), otherwise infinite
 	 * iteration can occur.
 	 */
-	if (size > dev->l2ad_end - dev->l2ad_start) {
-		cmn_err(CE_NOTE, "l2arc_write_max or l2arc_write_boost "
-		    "plus the overhead of log blocks (persistent L2ARC, "
-		    "%llu bytes) exceeds the size of the cache device "
-		    "(guid %llu), resetting them to the default (%d)",
-		    (u_longlong_t)l2arc_log_blk_overhead(size, dev),
-		    (u_longlong_t)dev->l2ad_vdev->vdev_guid, L2ARC_WRITE_SIZE);
-
-		size = l2arc_write_max = l2arc_write_boost = L2ARC_WRITE_SIZE;
-
-		if (l2arc_trim_ahead > 1) {
-			cmn_err(CE_NOTE, "l2arc_trim_ahead set to 1");
-			l2arc_trim_ahead = 1;
-		}
-
-		if (arc_warm == B_FALSE)
-			size += l2arc_write_boost;
-
-		size += l2arc_log_blk_overhead(size, dev);
-		if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0) {
-			size += MAX(64 * 1024 * 1024,
-			    (size * l2arc_trim_ahead) / 100);
-		}
-	}
+	size = MIN(size, (dev->l2ad_end - dev->l2ad_start) / 4);
 
 	return (size);