[PATCH v6 23/32] qcow2: Add subcluster support to discard_in_l2_slice()

[Alberto Garcia]

Two things need to be taken into account here:

1) With full_discard == true the L2 entry must be cleared completely.
   This also includes the L2 bitmap if the image has extended L2
   entries.

2) With full_discard == false we have to make the discarded cluster
   read back as zeroes. With normal L2 entries this is done with the
   QCOW_OFLAG_ZERO bit, whereas with extended L2 entries this is done
   with the individual 'all zeroes' bits for each subcluster.

   Note however that QCOW_OFLAG_ZERO is not supported in v2 qcow2
   images so, if there is a backing file, discard cannot guarantee
   that the image will read back as zeroes. If this is important for
   the caller it should forbid it as qcow2_co_pdiscard() does (see
   80f5c01183 for more details).

Signed-off-by: Alberto Garcia <address@hidden>
---
 block/qcow2-cluster.c | 51 +++++++++++++++++++------------------------
 1 file changed, 22 insertions(+), 29 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 4e59bbd545..5e70b76c21 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1847,11 +1847,16 @@ static int discard_in_l2_slice(BlockDriverState *bs, 
uint64_t offset,
     assert(nb_clusters <= INT_MAX);
 
     for (i = 0; i < nb_clusters; i++) {
-        uint64_t old_l2_entry;
-
-        old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        uint64_t old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
+        uint64_t old_l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
+        uint64_t new_l2_entry = old_l2_entry;
+        uint64_t new_l2_bitmap = old_l2_bitmap;
+        QCow2ClusterType type = qcow2_get_cluster_type(bs, old_l2_entry);
 
         /*
+         * If full_discard is true, the cluster should not read back as zeroes,
+         * but rather fall through to the backing file.
+         *
          * If full_discard is false, make sure that a discarded area reads back
          * as zeroes for v3 images (we cannot do it for v2 without actually
          * writing a zero-filled buffer). We can skip the operation if the
@@ -1860,40 +1865,28 @@ static int discard_in_l2_slice(BlockDriverState *bs, 
uint64_t offset,
          *
          * TODO We might want to use bdrv_block_status(bs) here, but we're
          * holding s->lock, so that doesn't work today.
-         *
-         * If full_discard is true, the sector should not read back as zeroes,
-         * but rather fall through to the backing file.
          */
-        switch (qcow2_get_cluster_type(bs, old_l2_entry)) {
-        case QCOW2_CLUSTER_UNALLOCATED:
-            if (full_discard || !bs->backing) {
-                continue;
+        if (full_discard) {
+            new_l2_entry = new_l2_bitmap = 0;
+        } else if (bs->backing || qcow2_cluster_is_allocated(type)) {
+            if (has_subclusters(s)) {
+                new_l2_entry = 0;
+                new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
+            } else {
+                new_l2_entry = s->qcow_version >= 3 ? QCOW_OFLAG_ZERO : 0;
             }
-            break;
+        }
 
-        case QCOW2_CLUSTER_ZERO_PLAIN:
-            if (!full_discard) {
-                continue;
-            }
-            break;
-
-        case QCOW2_CLUSTER_ZERO_ALLOC:
-        case QCOW2_CLUSTER_NORMAL:
-        case QCOW2_CLUSTER_COMPRESSED:
-            break;
-
-        default:
-            abort();
+        if (old_l2_entry == new_l2_entry && old_l2_bitmap == new_l2_bitmap) {
+            continue;
         }
 
         /* First remove L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
-        if (!full_discard && s->qcow_version >= 3) {
-            set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
-        } else {
-            set_l2_entry(s, l2_slice, l2_index + i, 0);
+        set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
+        if (has_subclusters(s)) {
+            set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
         }
-
         /* Then decrease the refcount */
         qcow2_free_any_clusters(bs, old_l2_entry, 1, type);
     }
-- 
2.20.1