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. Also take into account the
allocated size of log blocks when deciding when to stop writing buffers
to L2ARC.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes openzfs#14939

module/zfs/arc.c

@@ -965,7 +965,7 @@ static void l2arc_hdr_restore(const l2arc_log_ent_phys_t *le,
     l2arc_dev_t *dev);
 
 /* L2ARC persistence write I/O routines. */
-static void l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio,
+static uint64_t l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio,
     l2arc_write_callback_t *cb);
 
 /* L2ARC persistence auxiliary routines. */
@@ -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);
@@ -9413,8 +9418,14 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 			 * arcstat_l2_{size,asize} kstats are updated
 			 * internally.
 			 */
-			if (l2arc_log_blk_insert(dev, hdr))
-				l2arc_log_blk_commit(dev, pio, cb);
+			if (l2arc_log_blk_insert(dev, hdr)) {
+				/*
+				 * l2ad_hand has been accounted for in
+				 * l2arc_log_blk_commit().
+				 */
+				write_asize +=
+				    l2arc_log_blk_commit(dev, pio, cb);
+			}
 
 			zio_nowait(wzio);
 		}
@@ -10564,7 +10575,7 @@ l2arc_dev_hdr_update(l2arc_dev_t *dev)
  * This function allocates some memory to temporarily hold the serialized
  * buffer to be written. This is then released in l2arc_write_done.
  */
-static void
+static uint64_t
 l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
 {
 	l2arc_log_blk_phys_t	*lb = &dev->l2ad_log_blk;
@@ -10675,6 +10686,8 @@ l2arc_log_blk_commit(l2arc_dev_t *dev, zio_t *pio, l2arc_write_callback_t *cb)
 	dev->l2ad_log_ent_idx = 0;
 	dev->l2ad_log_blk_payload_asize = 0;
 	dev->l2ad_log_blk_payload_start = 0;
+
+	return (asize);
 }
 
 /*