[RFC PATCH v2 22/26] qcow2: Add subcluster support to handle_alloc_space()

[Alberto Garcia <berto@igalia.com>]

The bdrv_co_pwrite_zeroes() call here fills complete clusters with
zeroes, but it can happen that some subclusters are not part of the
write request or the copy-on-write. This patch makes sure that only
the affected subclusters are overwritten.

A potential improvement would be to also fill with zeroes the other
subclusters if we can guarantee that we are not overwriting existing
data. However this would waste more disk space, so we should first
evaluate if it's really worth doing.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 0261e87709..01322ca449 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2304,6 +2304,9 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
 
     for (m = l2meta; m != NULL; m = m->next) {
         int ret;
+        uint64_t start_offset = m->alloc_offset + m->cow_start.offset;
+        uint64_t nb_bytes = m->cow_end.offset + m->cow_end.nb_bytes -
+            m->cow_start.offset;
 
         if (!m->cow_start.nb_bytes && !m->cow_end.nb_bytes) {
             continue;
@@ -2318,16 +2321,14 @@ static int handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
          * efficiently zero out the whole clusters
          */
 
-        ret = qcow2_pre_write_overlap_check(bs, 0, m->alloc_offset,
-                                            m->nb_clusters * s->cluster_size,
+        ret = qcow2_pre_write_overlap_check(bs, 0, start_offset, nb_bytes,
                                             true);
         if (ret < 0) {
             return ret;
         }
 
         BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_SPACE);
-        ret = bdrv_co_pwrite_zeroes(s->data_file, m->alloc_offset,
-                                    m->nb_clusters * s->cluster_size,
+        ret = bdrv_co_pwrite_zeroes(s->data_file, start_offset, nb_bytes,
                                     BDRV_REQ_NO_FALLBACK);
         if (ret < 0) {
             if (ret != -ENOTSUP && ret != -EAGAIN) {
-- 
2.20.1