[PATCH 5/7] qcow2: zero_l2_subclusters: fall through to discard operation when requested

[Andrey Drobyshev]

When zeroizing subclusters within single cluster, detect usage of the
BDRV_REQ_MAY_UNMAP flag and fall through to the subcluster-based discard
operation, much like it's done with the cluster-based discards.  That
way subcluster-aligned operations "qemu-io -c 'write -z -u ...'" will
lead to actual unmap.

Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 block/qcow2-cluster.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index cf40f2dc12..040251f2c3 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -2242,7 +2242,7 @@ zero_l2_subclusters(BlockDriverState *bs, uint64_t offset,
                     unsigned nb_subclusters, int flags)
 {
     BDRVQcow2State *s = bs->opaque;
-    uint64_t new_l2_bitmap;
+    uint64_t new_l2_bitmap, l2_bitmap_mask;
     int ret, sc = offset_to_sc_index(s, offset);
     SubClusterRangeInfo scri = { 0 };
 
@@ -2251,9 +2251,10 @@ zero_l2_subclusters(BlockDriverState *bs, uint64_t 
offset,
         goto out;
     }
 
+    l2_bitmap_mask = QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);
     new_l2_bitmap = scri.l2_bitmap;
-    new_l2_bitmap |=  QCOW_OFLAG_SUB_ZERO_RANGE(sc, sc + nb_subclusters);
-    new_l2_bitmap &= ~QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);
+    new_l2_bitmap |= QCOW_OFLAG_SUB_ZERO_RANGE(sc, sc + nb_subclusters);
+    new_l2_bitmap &= ~l2_bitmap_mask;
 
     /*
      * If there're no non-zero subclusters left, we might as well zeroize
@@ -2266,6 +2267,16 @@ zero_l2_subclusters(BlockDriverState *bs, uint64_t 
offset,
                                 1, flags);
     }
 
+    /*
+     * If the request allows discarding subclusters and they're actually
+     * allocated, we go down the discard path since after the discard
+     * operation the subclusters are going to be read as zeroes anyway.
+     */
+    if ((flags & BDRV_REQ_MAY_UNMAP) && (scri.l2_bitmap & l2_bitmap_mask)) {
+        return discard_l2_subclusters(bs, offset, nb_subclusters,
+                                      QCOW2_DISCARD_REQUEST, false, &scri);
+    }
+
     if (new_l2_bitmap != scri.l2_bitmap) {
         set_l2_bitmap(s, scri.l2_slice, scri.l2_index, new_l2_bitmap);
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, scri.l2_slice);
-- 
2.39.3