[Qemu-devel] [PATCH v2] qcow2: Fix creation of large images

[Kevin Wolf <kwolf@redhat.com>]

qcow_create2 assumes that the new image will only need one cluster for its
refcount table initially. Obviously that's not true any more when the image is
big enough (exact value depends on the cluster size).

This patch calculates the refcount table size dynamically.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
v2:
- Made cluster calculation a bit more readable

 block/qcow2.c |   43 +++++++++++++++++++++++++++++++++----------
 1 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 80c99af..67affa6 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -850,10 +850,11 @@ static int qcow_create2(const char *filename, int64_t total_size,
 {
 
     int fd, header_size, backing_filename_len, l1_size, i, shift, l2_bits;
-    int ref_clusters, backing_format_len = 0;
+    int ref_clusters, reftable_clusters, backing_format_len = 0;
     int rounded_ext_bf_len = 0;
     QCowHeader header;
     uint64_t tmp, offset;
+    uint64_t old_ref_clusters;
     QCowCreateState s1, *s = &s1;
     QCowExtension ext_bf = {0, 0};
     int ret;
@@ -912,17 +913,37 @@ static int qcow_create2(const char *filename, int64_t total_size,
     header.l1_size = cpu_to_be32(l1_size);
     offset += align_offset(l1_size * sizeof(uint64_t), s->cluster_size);
 
-    s->refcount_table = qemu_mallocz(s->cluster_size);
+    /* count how many refcount blocks needed */
+
+#define NUM_CLUSTERS(bytes) \
+    (((bytes) + (s->cluster_size) - 1) / (s->cluster_size))
+
+    ref_clusters = NUM_CLUSTERS(NUM_CLUSTERS(offset) * sizeof(uint16_t));
+
+    do {
+        uint64_t image_clusters;
+        old_ref_clusters = ref_clusters;
+
+        /* Number of clusters used for the refcount table */
+        reftable_clusters = NUM_CLUSTERS(ref_clusters * sizeof(uint64_t));
+
+        /* Number of clusters that the whole image will have */
+        image_clusters = NUM_CLUSTERS(offset) + ref_clusters
+            + reftable_clusters;
+
+        /* Number of refcount blocks needed for the image */
+        ref_clusters = NUM_CLUSTERS(image_clusters * sizeof(uint16_t));
+
+    } while (ref_clusters != old_ref_clusters);
+
+    s->refcount_table = qemu_mallocz(reftable_clusters * s->cluster_size);
 
     s->refcount_table_offset = offset;
     header.refcount_table_offset = cpu_to_be64(offset);
-    header.refcount_table_clusters = cpu_to_be32(1);
-    offset += s->cluster_size;
+    header.refcount_table_clusters = cpu_to_be32(reftable_clusters);
+    offset += (reftable_clusters * s->cluster_size);
     s->refcount_block_offset = offset;
 
-    /* count how many refcount blocks needed */
-    tmp = offset >> s->cluster_bits;
-    ref_clusters = (tmp >> (s->cluster_bits - REFCOUNT_SHIFT)) + 1;
     for (i=0; i < ref_clusters; i++) {
         s->refcount_table[i] = cpu_to_be64(offset);
         offset += s->cluster_size;
@@ -934,7 +955,8 @@ static int qcow_create2(const char *filename, int64_t total_size,
     qcow2_create_refcount_update(s, 0, header_size);
     qcow2_create_refcount_update(s, s->l1_table_offset,
         l1_size * sizeof(uint64_t));
-    qcow2_create_refcount_update(s, s->refcount_table_offset, s->cluster_size);
+    qcow2_create_refcount_update(s, s->refcount_table_offset,
+        reftable_clusters * s->cluster_size);
     qcow2_create_refcount_update(s, s->refcount_block_offset,
         ref_clusters * s->cluster_size);
 
@@ -986,8 +1008,9 @@ static int qcow_create2(const char *filename, int64_t total_size,
         }
     }
     lseek(fd, s->refcount_table_offset, SEEK_SET);
-    ret = qemu_write_full(fd, s->refcount_table, s->cluster_size);
-    if (ret != s->cluster_size) {
+    ret = qemu_write_full(fd, s->refcount_table,
+        reftable_clusters * s->cluster_size);
+    if (ret != reftable_clusters * s->cluster_size) {
         ret = -errno;
         goto exit;
     }
-- 
1.6.6.1