vs_alloc can underflow in L2ARC vdevs

The current L2 ARC device code consistently uses psize to
increment vs_alloc but varies between psize and lsize when
decrementing it. The result of this behavior is that
vs_alloc can be decremented more that it is incremented
and underflow. This patch changes the code so asize is
used anywhere.

In addition, it ensures that vs_alloc gets incremented by
the L2 ARC device code as buffers are written and not at
the end of the l2arc_write_buffers() routine. The latter
(and old) way would temporarily underflow vs_alloc as
buffers that were just written, would be destroyed while
l2arc_write_buffers() was still looping.

Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Serapheim Dimitropoulos <[email protected]>
Closes #8298 

2 files changed, 16 insertions, 6 deletions

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index f5d94cbf9..385a2edec 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -3786,7 +3786,8 @@ arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr)
 {
 	l2arc_buf_hdr_t *l2hdr = &hdr->b_l2hdr;
 	l2arc_dev_t *dev = l2hdr->b_dev;
-	uint64_t psize = arc_hdr_size(hdr);
+	uint64_t psize = HDR_GET_PSIZE(hdr);
+	uint64_t asize = vdev_psize_to_asize(dev->l2ad_vdev, psize);
 
 	ASSERT(MUTEX_HELD(&dev->l2ad_mtx));
 	ASSERT(HDR_HAS_L2HDR(hdr));
@@ -3796,9 +3797,10 @@ arc_hdr_l2hdr_destroy(arc_buf_hdr_t *hdr)
 	ARCSTAT_INCR(arcstat_l2_psize, -psize);
 	ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));
 
-	vdev_space_update(dev->l2ad_vdev, -psize, 0, 0);
+	vdev_space_update(dev->l2ad_vdev, -asize, 0, 0);
 
-	(void) zfs_refcount_remove_many(&dev->l2ad_alloc, psize, hdr);
+	(void) zfs_refcount_remove_many(&dev->l2ad_alloc, arc_hdr_size(hdr),
+	    hdr);
 	arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
 }
 
@@ -8296,10 +8298,12 @@ top:
 			list_remove(buflist, hdr);
 			arc_hdr_clear_flags(hdr, ARC_FLAG_HAS_L2HDR);
 
-			ARCSTAT_INCR(arcstat_l2_psize, -arc_hdr_size(hdr));
+			uint64_t psize = HDR_GET_PSIZE(hdr);
+			ARCSTAT_INCR(arcstat_l2_psize, -psize);
 			ARCSTAT_INCR(arcstat_l2_lsize, -HDR_GET_LSIZE(hdr));
 
-			bytes_dropped += arc_hdr_size(hdr);
+			bytes_dropped +=
+			    vdev_psize_to_asize(dev->l2ad_vdev, psize);
 			(void) zfs_refcount_remove_many(&dev->l2ad_alloc,
 			    arc_hdr_size(hdr), hdr);
 		}
@@ -9015,6 +9019,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 			write_psize += psize;
 			write_asize += asize;
 			dev->l2ad_hand += asize;
+			vdev_space_update(dev->l2ad_vdev, asize, 0, 0);
 
 			mutex_exit(hash_lock);
 
@@ -9040,7 +9045,6 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 	ARCSTAT_INCR(arcstat_l2_write_bytes, write_psize);
 	ARCSTAT_INCR(arcstat_l2_lsize, write_lsize);
 	ARCSTAT_INCR(arcstat_l2_psize, write_psize);
-	vdev_space_update(dev->l2ad_vdev, write_psize, 0, 0);
 
 	/*
 	 * Bump device hand to the device start if it is approaching the end.
diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index 50d230ccb..def98ad13 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -4162,6 +4162,11 @@ vdev_space_update(vdev_t *vd, int64_t alloc_delta, int64_t defer_delta,
 	dspace_delta = vdev_deflated_space(vd, space_delta);
 
 	mutex_enter(&vd->vdev_stat_lock);
+	/* ensure we won't underflow */
+	if (alloc_delta < 0) {
+		ASSERT3U(vd->vdev_stat.vs_alloc, >=, -alloc_delta);
+	}
+
 	vd->vdev_stat.vs_alloc += alloc_delta;
 	vd->vdev_stat.vs_space += space_delta;
 	vd->vdev_stat.vs_dspace += dspace_delta;
@@ -4169,6 +4174,7 @@ vdev_space_update(vdev_t *vd, int64_t alloc_delta, int64_t defer_delta,
 
 	/* every class but log contributes to root space stats */
 	if (vd->vdev_mg != NULL && !vd->vdev_islog) {
+		ASSERT(!vd->vdev_isl2cache);
 		mutex_enter(&rvd->vdev_stat_lock);
 		rvd->vdev_stat.vs_alloc += alloc_delta;
 		rvd->vdev_stat.vs_space += space_delta;