qcow2: Discard/zero clusters by byte count

Passing a byte offset, but sector count, when we ultimately
want to operate on cluster granularity, is madness.  Clean up
the external interfaces to take both offset and count as bytes,
while still keeping the assertion added previously that the
caller must align the values to a cluster.  Then rename things
to make sure backports don't get confused by changed units:
instead of qcow2_discard_clusters() and qcow2_zero_clusters(),
we now have qcow2_cluster_discard() and qcow2_cluster_zeroize().

The internal functions still operate on clusters at a time, and
return an int for number of cleared clusters; but on an image
with 2M clusters, a single L2 table holds 256k entries that each
represent a 2M cluster, totalling well over INT_MAX bytes if we
ever had a request for that many bytes at once.  All our callers
currently limit themselves to 32-bit bytes (and therefore fewer
clusters), but by making this function 64-bit clean, we have one
less place to clean up if we later improve the block layer to
support 64-bit bytes through all operations (with the block layer
auto-fragmenting on behalf of more-limited drivers), rather than
the current state where some interfaces are artificially limited
to INT_MAX at a time.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170507000552.20847-13-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 22 insertions, 20 deletions

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 43bde569a7..347d94b0d2 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1549,34 +1549,36 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
     return nb_clusters;
 }
 
-int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
-    int nb_sectors, enum qcow2_discard_type type, bool full_discard)
+int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset,
+                          uint64_t bytes, enum qcow2_discard_type type,
+                          bool full_discard)
 {
     BDRVQcow2State *s = bs->opaque;
-    uint64_t end_offset;
+    uint64_t end_offset = offset + bytes;
     uint64_t nb_clusters;
+    int64_t cleared;
     int ret;
 
-    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
-
     /* Caller must pass aligned values, except at image end */
     assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
     assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
            end_offset == bs->total_sectors << BDRV_SECTOR_BITS);
 
-    nb_clusters = size_to_clusters(s, end_offset - offset);
+    nb_clusters = size_to_clusters(s, bytes);
 
     s->cache_discards = true;
 
     /* Each L2 table is handled by its own loop iteration */
     while (nb_clusters > 0) {
-        ret = discard_single_l2(bs, offset, nb_clusters, type, full_discard);
-        if (ret < 0) {
+        cleared = discard_single_l2(bs, offset, nb_clusters, type,
+                                    full_discard);
+        if (cleared < 0) {
+            ret = cleared;
             goto fail;
         }
 
-        nb_clusters -= ret;
-        offset += (ret * s->cluster_size);
+        nb_clusters -= cleared;
+        offset += (cleared * s->cluster_size);
     }
 
     ret = 0;
@@ -1641,16 +1643,15 @@ static int zero_single_l2(BlockDriverState *bs, uint64_t offset,
     return nb_clusters;
 }
 
-int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors,
-                        int flags)
+int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
+                          uint64_t bytes, int flags)
 {
     BDRVQcow2State *s = bs->opaque;
-    uint64_t end_offset;
+    uint64_t end_offset = offset + bytes;
     uint64_t nb_clusters;
+    int64_t cleared;
     int ret;
 
-    end_offset = offset + (nb_sectors << BDRV_SECTOR_BITS);
-
     /* Caller must pass aligned values, except at image end */
     assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
     assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
@@ -1662,18 +1663,19 @@ int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors,
     }
 
     /* Each L2 table is handled by its own loop iteration */
-    nb_clusters = size_to_clusters(s, nb_sectors << BDRV_SECTOR_BITS);
+    nb_clusters = size_to_clusters(s, bytes);
 
     s->cache_discards = true;
 
     while (nb_clusters > 0) {
-        ret = zero_single_l2(bs, offset, nb_clusters, flags);
-        if (ret < 0) {
+        cleared = zero_single_l2(bs, offset, nb_clusters, flags);
+        if (cleared < 0) {
+            ret = cleared;
             goto fail;
         }
 
-        nb_clusters -= ret;
-        offset += (ret * s->cluster_size);
+        nb_clusters -= cleared;
+        offset += (cleared * s->cluster_size);
     }
 
     ret = 0;