[Qemu-devel] [PATCH 2/2] qcow2: Implement bdrv_truncate() for growing images

[Stefan Hajnoczi]

This patch adds the ability to grow qcow2 images in-place using
bdrv_truncate().  This enables qemu-img resize command support for
qcow2.

Snapshots are not supported and bdrv_truncate() will return -ENOTSUP.
The notion of resizing an image with snapshots could lead to confusion:
users may expect snapshots to remain unchanged, but this is not possible
with the current qcow2 on-disk format where the header.size field is
global instead of per-snapshot.  Others may expect snapshots to change
size along with the current image data.  I think it is safest to not
support snapshots and perhaps add behavior later if there is a
consensus.

Backing images continue to work.  If the image is now larger than its
backing image, zeroes are read when accessing beyond the end of the
backing image.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
This applies to kevin/block.

 block/qcow2-cluster.c  |   64 ++++++++++++++++++++++++++++++++++++++---------
 block/qcow2-snapshot.c |    2 +-
 block/qcow2.c          |   43 +++++++++++++++++++++++++++++---
 block/qcow2.h          |    9 ++++++-
 4 files changed, 99 insertions(+), 19 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index c11680d..20c8426 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -28,30 +28,39 @@
 #include "block_int.h"
 #include "block/qcow2.h"
 
-int qcow2_grow_l1_table(BlockDriverState *bs, int min_size)
+/*
+ * qcow2_grow_l1_table_common
+ *
+ * Grows the L1 table and updates the header on disk.
+ *
+ * Setting new_l1_vm_state_index to s->l1_vm_state_index grows the vm state
+ * area.
+ *
+ * Setting new_l1_vm_state_index to the new end of image grows the image data
+ * area.
+ *
+ * Returns 0 on success, -errno in failure case.
+ */
+static int qcow2_grow_l1_table_common(BlockDriverState *bs,
+                                      int new_l1_vm_state_index,
+                                      int new_l1_size)
 {
     BDRVQcowState *s = bs->opaque;
-    int new_l1_size, new_l1_size2, ret, i;
+    int new_l1_size2, ret, i;
     uint64_t *new_l1_table;
     int64_t new_l1_table_offset;
     uint8_t data[12];
 
-    new_l1_size = s->l1_size;
-    if (min_size <= new_l1_size)
-        return 0;
-    if (new_l1_size == 0) {
-        new_l1_size = 1;
-    }
-    while (min_size > new_l1_size) {
-        new_l1_size = (new_l1_size * 3 + 1) / 2;
-    }
 #ifdef DEBUG_ALLOC2
     printf("grow l1_table from %d to %d\n", s->l1_size, new_l1_size);
 #endif
 
     new_l1_size2 = sizeof(uint64_t) * new_l1_size;
     new_l1_table = qemu_mallocz(align_offset(new_l1_size2, 512));
-    memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
+    memcpy(new_l1_table, s->l1_table, s->l1_vm_state_index * sizeof(uint64_t));
+    memcpy(&new_l1_table[new_l1_vm_state_index],
+           &s->l1_table[s->l1_vm_state_index],
+           (s->l1_size - s->l1_vm_state_index) * sizeof(uint64_t));
 
     /* write new table (align to cluster) */
     BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
@@ -83,6 +92,7 @@ int qcow2_grow_l1_table(BlockDriverState *bs, int min_size)
     s->l1_table_offset = new_l1_table_offset;
     s->l1_table = new_l1_table;
     s->l1_size = new_l1_size;
+    s->l1_vm_state_index = new_l1_vm_state_index;
     return 0;
  fail:
     qemu_free(new_l1_table);
@@ -90,6 +100,34 @@ int qcow2_grow_l1_table(BlockDriverState *bs, int min_size)
     return ret < 0 ? ret : -EIO;
 }
 
+int qcow2_grow_l1_vm_state(BlockDriverState *bs, int min_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_l1_size;
+
+    new_l1_size = s->l1_size;
+    if (min_size <= new_l1_size)
+        return 0;
+    if (new_l1_size == 0) {
+        new_l1_size = 1;
+    }
+    while (min_size > new_l1_size) {
+        new_l1_size = (new_l1_size * 3 + 1) / 2;
+    }
+
+    return qcow2_grow_l1_table_common(bs, s->l1_vm_state_index, new_l1_size);
+}
+
+int qcow2_grow_l1_image_data(BlockDriverState *bs, int new_l1_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_l1_vm_state_index;
+
+    new_l1_vm_state_index = new_l1_size;
+    new_l1_size += s->l1_size - s->l1_vm_state_index;
+    return qcow2_grow_l1_table_common(bs, new_l1_vm_state_index, new_l1_size);
+}
+
 void qcow2_l2_cache_reset(BlockDriverState *bs)
 {
     BDRVQcowState *s = bs->opaque;
@@ -524,7 +562,7 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t 
offset,
 
     l1_index = offset >> (s->l2_bits + s->cluster_bits);
     if (l1_index >= s->l1_size) {
-        ret = qcow2_grow_l1_table(bs, l1_index + 1);
+        ret = qcow2_grow_l1_vm_state(bs, l1_index + 1);
         if (ret < 0) {
             return ret;
         }
diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
index 2a21c17..7f0d810 100644
--- a/block/qcow2-snapshot.c
+++ b/block/qcow2-snapshot.c
@@ -326,7 +326,7 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char 
*snapshot_id)
     if (qcow2_update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, -1) 
< 0)
         goto fail;
 
-    if (qcow2_grow_l1_table(bs, sn->l1_size) < 0)
+    if (qcow2_grow_l1_vm_state(bs, sn->l1_size) < 0)
         goto fail;
 
     s->l1_size = sn->l1_size;
diff --git a/block/qcow2.c b/block/qcow2.c
index 4a7ab66..ab622a2 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -140,7 +140,7 @@ static int qcow_read_extensions(BlockDriverState *bs, 
uint64_t start_offset,
 static int qcow_open(BlockDriverState *bs, int flags)
 {
     BDRVQcowState *s = bs->opaque;
-    int len, i, shift;
+    int len, i;
     QCowHeader header;
     uint64_t ext_end;
 
@@ -188,8 +188,7 @@ static int qcow_open(BlockDriverState *bs, int flags)
 
     /* read the level 1 table */
     s->l1_size = header.l1_size;
-    shift = s->cluster_bits + s->l2_bits;
-    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
+    s->l1_vm_state_index = size_to_l1(s, header.size);
     /* the L1 table must contain at least enough entries to put
        header.size bytes */
     if (s->l1_size < s->l1_vm_state_index)
@@ -1095,6 +1094,40 @@ static int qcow_make_empty(BlockDriverState *bs)
     return 0;
 }
 
+static int qcow2_truncate(BlockDriverState *bs, int64_t offset)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, new_l1_size;
+
+    if (offset & 511) {
+        return -EINVAL;
+    }
+
+    /* cannot proceed if image has snapshots */
+    if (s->nb_snapshots) {
+        return -ENOTSUP;
+    }
+
+    /* shrinking is currently not supported */
+    if (offset < bs->total_sectors << BDRV_SECTOR_BITS) {
+        return -ENOTSUP;
+    }
+
+    new_l1_size = size_to_l1(s, offset);
+    ret = qcow2_grow_l1_image_data(bs, new_l1_size);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /* write updated header.size */
+    offset = cpu_to_be64(offset);
+    if (bdrv_pwrite(bs->file, offsetof(QCowHeader, size), &offset,
+                    sizeof(uint64_t)) != sizeof(uint64_t)) {
+        return -EIO;
+    }
+    return 0;
+}
+
 /* XXX: put compressed sectors first, then all the cluster aligned
    tables to avoid losing bytes in alignment */
 static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
@@ -1294,7 +1327,9 @@ static BlockDriver bdrv_qcow2 = {
     .bdrv_aio_readv    = qcow_aio_readv,
     .bdrv_aio_writev   = qcow_aio_writev,
     .bdrv_aio_flush    = qcow_aio_flush,
-    .bdrv_write_compressed = qcow_write_compressed,
+
+    .bdrv_truncate          = qcow2_truncate,
+    .bdrv_write_compressed  = qcow_write_compressed,
 
     .bdrv_snapshot_create   = qcow2_snapshot_create,
     .bdrv_snapshot_goto     = qcow2_snapshot_goto,
diff --git a/block/qcow2.h b/block/qcow2.h
index 5bd08db..c328248 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -150,6 +150,12 @@ static inline int size_to_clusters(BDRVQcowState *s, 
int64_t size)
     return (size + (s->cluster_size - 1)) >> s->cluster_bits;
 }
 
+static inline int size_to_l1(BDRVQcowState *s, int64_t size)
+{
+    int shift = s->cluster_bits + s->l2_bits;
+    return (size + (1ULL << shift) - 1) >> shift;
+}
+
 static inline int64_t align_offset(int64_t offset, int n)
 {
     offset = (offset + n - 1) & ~(n - 1);
@@ -182,7 +188,8 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
 int qcow2_check_refcounts(BlockDriverState *bs);
 
 /* qcow2-cluster.c functions */
-int qcow2_grow_l1_table(BlockDriverState *bs, int min_size);
+int qcow2_grow_l1_vm_state(BlockDriverState *bs, int min_size);
+int qcow2_grow_l1_image_data(BlockDriverState *bs, int new_l1_size);
 void qcow2_l2_cache_reset(BlockDriverState *bs);
 int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset);
 void qcow2_encrypt_sectors(BDRVQcowState *s, int64_t sector_num,
-- 
1.7.0