Fix the L2ARC write size calculating logic

l2arc_write_size() should return the write size after adjusting for trim
and overhead of the L2ARC log blocks.

Signed-off-by: George Amanakis <gamanakis@gmail.com>

module/zfs/arc.c

@@ -8175,7 +8175,7 @@ l2arc_write_eligible(uint64_t spa_guid, arc_buf_hdr_t *hdr)
 static uint64_t
 l2arc_write_size(l2arc_dev_t *dev)
 {
-	uint64_t size, dev_size, tsize;
+	uint64_t size;
 
 	/*
 	 * Make sure our globals have meaningful values in case the user
@@ -8192,35 +8192,40 @@ l2arc_write_size(l2arc_dev_t *dev)
 	if (arc_warm == B_FALSE)
 		size += l2arc_write_boost;
 
-	/*
-	 * Make sure the write size does not exceed the size of the cache
-	 * device. This is important in l2arc_evict(), otherwise infinite
-	 * iteration can occur.
-	 */
-	dev_size = dev->l2ad_end - dev->l2ad_start;
-
 	/* We need to add in the worst case scenario of log block overhead. */
-	tsize = size + l2arc_log_blk_overhead(size, dev);
+	size += l2arc_log_blk_overhead(size, dev);
 	if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0) {
 		/*
 		 * Trim ahead of the write size 64MB or (l2arc_trim_ahead/100)
 		 * times the writesize, whichever is greater.
 		 */
-		tsize += MAX(64 * 1024 * 1024,
-		    (tsize * l2arc_trim_ahead) / 100);
+		size += MAX(64 * 1024 * 1024,
+		    (size * l2arc_trim_ahead) / 100);
 	}
 
-	if (tsize >= dev_size) {
+	/*
+	 * Make sure the write size does not exceed the size of the cache
+	 * 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 (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);
+		}
 	}
 
 	return (size);