qcow2: Split out refcount handling

qcow2-refcount.c contains all functions which are related to cluster
allocation and management in the image file. A large part of this is the
reference counting of these clusters.

Also a header file qcow2.h is introduced which will contain the interface of
the split qcow2 modules.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 808 insertions, 0 deletions

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
new file mode 100644
index 000000000..b70afe963
--- /dev/null
+++ b/block/qcow2-refcount.c
@@ -0,0 +1,808 @@
+/*
+ * Block driver for the QCOW version 2 format
+ *
+ * Copyright (c) 2004-2006 Fabrice Bellard
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu-common.h"
+#include "block_int.h"
+#include "block/qcow2.h"
+
+static int64_t alloc_clusters_noref(BlockDriverState *bs, int64_t size);
+static int update_refcount(BlockDriverState *bs,
+                            int64_t offset, int64_t length,
+                            int addend);
+
+/*********************************************************/
+/* refcount handling */
+
+int refcount_init(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, refcount_table_size2, i;
+
+    s->refcount_block_cache = qemu_malloc(s->cluster_size);
+    refcount_table_size2 = s->refcount_table_size * sizeof(uint64_t);
+    s->refcount_table = qemu_malloc(refcount_table_size2);
+    if (s->refcount_table_size > 0) {
+        ret = bdrv_pread(s->hd, s->refcount_table_offset,
+                         s->refcount_table, refcount_table_size2);
+        if (ret != refcount_table_size2)
+            goto fail;
+        for(i = 0; i < s->refcount_table_size; i++)
+            be64_to_cpus(&s->refcount_table[i]);
+    }
+    return 0;
+ fail:
+    return -ENOMEM;
+}
+
+void refcount_close(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    qemu_free(s->refcount_block_cache);
+    qemu_free(s->refcount_table);
+}
+
+
+static int load_refcount_block(BlockDriverState *bs,
+                               int64_t refcount_block_offset)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+    ret = bdrv_pread(s->hd, refcount_block_offset, s->refcount_block_cache,
+                     s->cluster_size);
+    if (ret != s->cluster_size)
+        return -EIO;
+    s->refcount_block_cache_offset = refcount_block_offset;
+    return 0;
+}
+
+static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
+{
+    BDRVQcowState *s = bs->opaque;
+    int refcount_table_index, block_index;
+    int64_t refcount_block_offset;
+
+    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    if (refcount_table_index >= s->refcount_table_size)
+        return 0;
+    refcount_block_offset = s->refcount_table[refcount_table_index];
+    if (!refcount_block_offset)
+        return 0;
+    if (refcount_block_offset != s->refcount_block_cache_offset) {
+        /* better than nothing: return allocated if read error */
+        if (load_refcount_block(bs, refcount_block_offset) < 0)
+            return 1;
+    }
+    block_index = cluster_index &
+        ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+    return be16_to_cpu(s->refcount_block_cache[block_index]);
+}
+
+static int grow_refcount_table(BlockDriverState *bs, int min_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_table_size, new_table_size2, refcount_table_clusters, i, ret;
+    uint64_t *new_table;
+    int64_t table_offset;
+    uint8_t data[12];
+    int old_table_size;
+    int64_t old_table_offset;
+
+    if (min_size <= s->refcount_table_size)
+        return 0;
+    /* compute new table size */
+    refcount_table_clusters = s->refcount_table_size >> (s->cluster_bits - 3);
+    for(;;) {
+        if (refcount_table_clusters == 0) {
+            refcount_table_clusters = 1;
+        } else {
+            refcount_table_clusters = (refcount_table_clusters * 3 + 1) / 2;
+        }
+        new_table_size = refcount_table_clusters << (s->cluster_bits - 3);
+        if (min_size <= new_table_size)
+            break;
+    }
+#ifdef DEBUG_ALLOC2
+    printf("grow_refcount_table from %d to %d\n",
+           s->refcount_table_size,
+           new_table_size);
+#endif
+    new_table_size2 = new_table_size * sizeof(uint64_t);
+    new_table = qemu_mallocz(new_table_size2);
+    memcpy(new_table, s->refcount_table,
+           s->refcount_table_size * sizeof(uint64_t));
+    for(i = 0; i < s->refcount_table_size; i++)
+        cpu_to_be64s(&new_table[i]);
+    /* Note: we cannot update the refcount now to avoid recursion */
+    table_offset = alloc_clusters_noref(bs, new_table_size2);
+    ret = bdrv_pwrite(s->hd, table_offset, new_table, new_table_size2);
+    if (ret != new_table_size2)
+        goto fail;
+    for(i = 0; i < s->refcount_table_size; i++)
+        be64_to_cpus(&new_table[i]);
+
+    cpu_to_be64w((uint64_t*)data, table_offset);
+    cpu_to_be32w((uint32_t*)(data + 8), refcount_table_clusters);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, refcount_table_offset),
+                    data, sizeof(data)) != sizeof(data))
+        goto fail;
+    qemu_free(s->refcount_table);
+    old_table_offset = s->refcount_table_offset;
+    old_table_size = s->refcount_table_size;
+    s->refcount_table = new_table;
+    s->refcount_table_size = new_table_size;
+    s->refcount_table_offset = table_offset;
+
+    update_refcount(bs, table_offset, new_table_size2, 1);
+    free_clusters(bs, old_table_offset, old_table_size * sizeof(uint64_t));
+    return 0;
+ fail:
+    free_clusters(bs, table_offset, new_table_size2);
+    qemu_free(new_table);
+    return -EIO;
+}
+
+
+static int64_t alloc_refcount_block(BlockDriverState *bs, int64_t cluster_index)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t offset, refcount_block_offset;
+    int ret, refcount_table_index;
+    uint64_t data64;
+
+    /* Find L1 index and grow refcount table if needed */
+    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    if (refcount_table_index >= s->refcount_table_size) {
+        ret = grow_refcount_table(bs, refcount_table_index + 1);
+        if (ret < 0)
+            return ret;
+    }
+
+    /* Load or allocate the refcount block */
+    refcount_block_offset = s->refcount_table[refcount_table_index];
+    if (!refcount_block_offset) {
+        /* create a new refcount block */
+        /* Note: we cannot update the refcount now to avoid recursion */
+        offset = alloc_clusters_noref(bs, s->cluster_size);
+        memset(s->refcount_block_cache, 0, s->cluster_size);
+        ret = bdrv_pwrite(s->hd, offset, s->refcount_block_cache, s->cluster_size);
+        if (ret != s->cluster_size)
+            return -EINVAL;
+        s->refcount_table[refcount_table_index] = offset;
+        data64 = cpu_to_be64(offset);
+        ret = bdrv_pwrite(s->hd, s->refcount_table_offset +
+                          refcount_table_index * sizeof(uint64_t),
+                          &data64, sizeof(data64));
+        if (ret != sizeof(data64))
+            return -EINVAL;
+
+        refcount_block_offset = offset;
+        s->refcount_block_cache_offset = offset;
+        update_refcount(bs, offset, s->cluster_size, 1);
+    } else {
+        if (refcount_block_offset != s->refcount_block_cache_offset) {
+            if (load_refcount_block(bs, refcount_block_offset) < 0)
+                return -EIO;
+        }
+    }
+
+    return refcount_block_offset;
+}
+
+/* XXX: cache several refcount block clusters ? */
+static int update_refcount(BlockDriverState *bs,
+                            int64_t offset, int64_t length,
+                            int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t start, last, cluster_offset;
+    int64_t refcount_block_offset = 0;
+    int64_t table_index = -1, old_table_index;
+    int first_index = -1, last_index = -1;
+
+#ifdef DEBUG_ALLOC2
+    printf("update_refcount: offset=%lld size=%lld addend=%d\n",
+           offset, length, addend);
+#endif
+    if (length <= 0)
+        return -EINVAL;
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + length - 1) & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last;
+        cluster_offset += s->cluster_size)
+    {
+        int block_index, refcount;
+        int64_t cluster_index = cluster_offset >> s->cluster_bits;
+
+        /* Only write refcount block to disk when we are done with it */
+        old_table_index = table_index;
+        table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+        if ((old_table_index >= 0) && (table_index != old_table_index)) {
+            size_t size = (last_index - first_index + 1) << REFCOUNT_SHIFT;
+            if (bdrv_pwrite(s->hd,
+                refcount_block_offset + (first_index << REFCOUNT_SHIFT),
+                &s->refcount_block_cache[first_index], size) != size)
+            {
+                return -EIO;
+            }
+
+            first_index = -1;
+            last_index = -1;
+        }
+
+        /* Load the refcount block and allocate it if needed */
+        refcount_block_offset = alloc_refcount_block(bs, cluster_index);
+        if (refcount_block_offset < 0) {
+            return refcount_block_offset;
+        }
+
+        /* we can update the count and save it */
+        block_index = cluster_index &
+            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+        if (first_index == -1 || block_index < first_index) {
+            first_index = block_index;
+        }
+        if (block_index > last_index) {
+            last_index = block_index;
+        }
+
+        refcount = be16_to_cpu(s->refcount_block_cache[block_index]);
+        refcount += addend;
+        if (refcount < 0 || refcount > 0xffff)
+            return -EINVAL;
+        if (refcount == 0 && cluster_index < s->free_cluster_index) {
+            s->free_cluster_index = cluster_index;
+        }
+        s->refcount_block_cache[block_index] = cpu_to_be16(refcount);
+    }
+
+    /* Write last changed block to disk */
+    if (refcount_block_offset != 0) {
+        size_t size = (last_index - first_index + 1) << REFCOUNT_SHIFT;
+        if (bdrv_pwrite(s->hd,
+            refcount_block_offset + (first_index << REFCOUNT_SHIFT),
+            &s->refcount_block_cache[first_index], size) != size)
+        {
+            return -EIO;
+        }
+    }
+
+    return 0;
+}
+
+/* addend must be 1 or -1 */
+static int update_cluster_refcount(BlockDriverState *bs,
+                                   int64_t cluster_index,
+                                   int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+
+    ret = update_refcount(bs, cluster_index << s->cluster_bits, 1, addend);
+    if (ret < 0) {
+        return ret;
+    }
+
+    return get_refcount(bs, cluster_index);
+}
+
+
+
+/*********************************************************/
+/* cluster allocation functions */
+
+
+
+/* return < 0 if error */
+static int64_t alloc_clusters_noref(BlockDriverState *bs, int64_t size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i, nb_clusters;
+
+    nb_clusters = size_to_clusters(s, size);
+retry:
+    for(i = 0; i < nb_clusters; i++) {
+        int64_t i = s->free_cluster_index++;
+        if (get_refcount(bs, i) != 0)
+            goto retry;
+    }
+#ifdef DEBUG_ALLOC2
+    printf("alloc_clusters: size=%lld -> %lld\n",
+            size,
+            (s->free_cluster_index - nb_clusters) << s->cluster_bits);
+#endif
+    return (s->free_cluster_index - nb_clusters) << s->cluster_bits;
+}
+
+int64_t alloc_clusters(BlockDriverState *bs, int64_t size)
+{
+    int64_t offset;
+
+    offset = alloc_clusters_noref(bs, size);
+    update_refcount(bs, offset, size, 1);
+    return offset;
+}
+
+/* only used to allocate compressed sectors. We try to allocate
+   contiguous sectors. size must be <= cluster_size */
+int64_t alloc_bytes(BlockDriverState *bs, int size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t offset, cluster_offset;
+    int free_in_cluster;
+
+    assert(size > 0 && size <= s->cluster_size);
+    if (s->free_byte_offset == 0) {
+        s->free_byte_offset = alloc_clusters(bs, s->cluster_size);
+    }
+ redo:
+    free_in_cluster = s->cluster_size -
+        (s->free_byte_offset & (s->cluster_size - 1));
+    if (size <= free_in_cluster) {
+        /* enough space in current cluster */
+        offset = s->free_byte_offset;
+        s->free_byte_offset += size;
+        free_in_cluster -= size;
+        if (free_in_cluster == 0)
+            s->free_byte_offset = 0;
+        if ((offset & (s->cluster_size - 1)) != 0)
+            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
+    } else {
+        offset = alloc_clusters(bs, s->cluster_size);
+        cluster_offset = s->free_byte_offset & ~(s->cluster_size - 1);
+        if ((cluster_offset + s->cluster_size) == offset) {
+            /* we are lucky: contiguous data */
+            offset = s->free_byte_offset;
+            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
+            s->free_byte_offset += size;
+        } else {
+            s->free_byte_offset = offset;
+            goto redo;
+        }
+    }
+    return offset;
+}
+
+void free_clusters(BlockDriverState *bs,
+                          int64_t offset, int64_t size)
+{
+    update_refcount(bs, offset, size, -1);
+}
+
+
+
+/*********************************************************/
+/* snapshots and image creation */
+
+
+
+void create_refcount_update(QCowCreateState *s, int64_t offset, int64_t size)
+{
+    int refcount;
+    int64_t start, last, cluster_offset;
+    uint16_t *p;
+
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + size - 1)  & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last;
+        cluster_offset += s->cluster_size) {
+        p = &s->refcount_block[cluster_offset >> s->cluster_bits];
+        refcount = be16_to_cpu(*p);
+        refcount++;
+        *p = cpu_to_be16(refcount);
+    }
+}
+
+/* update the refcounts of snapshots and the copied flag */
+int update_snapshot_refcount(BlockDriverState *bs,
+                             int64_t l1_table_offset,
+                             int l1_size,
+                             int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2, l1_allocated;
+    int64_t old_offset, old_l2_offset;
+    int l2_size, i, j, l1_modified, l2_modified, nb_csectors, refcount;
+
+    l2_cache_reset(bs);
+
+    l2_table = NULL;
+    l1_table = NULL;
+    l1_size2 = l1_size * sizeof(uint64_t);
+    l1_allocated = 0;
+    if (l1_table_offset != s->l1_table_offset) {
+        l1_table = qemu_malloc(l1_size2);
+        l1_allocated = 1;
+        if (bdrv_pread(s->hd, l1_table_offset,
+                       l1_table, l1_size2) != l1_size2)
+            goto fail;
+        for(i = 0;i < l1_size; i++)
+            be64_to_cpus(&l1_table[i]);
+    } else {
+        assert(l1_size == s->l1_size);
+        l1_table = s->l1_table;
+        l1_allocated = 0;
+    }
+
+    l2_size = s->l2_size * sizeof(uint64_t);
+    l2_table = qemu_malloc(l2_size);
+    l1_modified = 0;
+    for(i = 0; i < l1_size; i++) {
+        l2_offset = l1_table[i];
+        if (l2_offset) {
+            old_l2_offset = l2_offset;
+            l2_offset &= ~QCOW_OFLAG_COPIED;
+            l2_modified = 0;
+            if (bdrv_pread(s->hd, l2_offset, l2_table, l2_size) != l2_size)
+                goto fail;
+            for(j = 0; j < s->l2_size; j++) {
+                offset = be64_to_cpu(l2_table[j]);
+                if (offset != 0) {
+                    old_offset = offset;
+                    offset &= ~QCOW_OFLAG_COPIED;
+                    if (offset & QCOW_OFLAG_COMPRESSED) {
+                        nb_csectors = ((offset >> s->csize_shift) &
+                                       s->csize_mask) + 1;
+                        if (addend != 0)
+                            update_refcount(bs, (offset & s->cluster_offset_mask) & ~511,
+                                            nb_csectors * 512, addend);
+                        /* compressed clusters are never modified */
+                        refcount = 2;
+                    } else {
+                        if (addend != 0) {
+                            refcount = update_cluster_refcount(bs, offset >> s->cluster_bits, addend);
+                        } else {
+                            refcount = get_refcount(bs, offset >> s->cluster_bits);
+                        }
+                    }
+
+                    if (refcount == 1) {
+                        offset |= QCOW_OFLAG_COPIED;
+                    }
+                    if (offset != old_offset) {
+                        l2_table[j] = cpu_to_be64(offset);
+                        l2_modified = 1;
+                    }
+                }
+            }
+            if (l2_modified) {
+                if (bdrv_pwrite(s->hd,
+                                l2_offset, l2_table, l2_size) != l2_size)
+                    goto fail;
+            }
+
+            if (addend != 0) {
+                refcount = update_cluster_refcount(bs, l2_offset >> s->cluster_bits, addend);
+            } else {
+                refcount = get_refcount(bs, l2_offset >> s->cluster_bits);
+            }
+            if (refcount == 1) {
+                l2_offset |= QCOW_OFLAG_COPIED;
+            }
+            if (l2_offset != old_l2_offset) {
+                l1_table[i] = l2_offset;
+                l1_modified = 1;
+            }
+        }
+    }
+    if (l1_modified) {
+        for(i = 0; i < l1_size; i++)
+            cpu_to_be64s(&l1_table[i]);
+        if (bdrv_pwrite(s->hd, l1_table_offset, l1_table,
+                        l1_size2) != l1_size2)
+            goto fail;
+        for(i = 0; i < l1_size; i++)
+            be64_to_cpus(&l1_table[i]);
+    }
+    if (l1_allocated)
+        qemu_free(l1_table);
+    qemu_free(l2_table);
+    return 0;
+ fail:
+    if (l1_allocated)
+        qemu_free(l1_table);
+    qemu_free(l2_table);
+    return -EIO;
+}
+
+
+
+
+/*********************************************************/
+/* refcount checking functions */
+
+
+
+/*
+ * Increases the refcount for a range of clusters in a given refcount table.
+ * This is used to construct a temporary refcount table out of L1 and L2 tables
+ * which can be compared the the refcount table saved in the image.
+ *
+ * Returns the number of errors in the image that were found
+ */
+static int inc_refcounts(BlockDriverState *bs,
+                          uint16_t *refcount_table,
+                          int refcount_table_size,
+                          int64_t offset, int64_t size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t start, last, cluster_offset;
+    int k;
+    int errors = 0;
+
+    if (size <= 0)
+        return 0;
+
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + size - 1) & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last;
+        cluster_offset += s->cluster_size) {
+        k = cluster_offset >> s->cluster_bits;
+        if (k < 0 || k >= refcount_table_size) {
+            fprintf(stderr, "ERROR: invalid cluster offset=0x%" PRIx64 "\n",
+                cluster_offset);
+            errors++;
+        } else {
+            if (++refcount_table[k] == 0) {
+                fprintf(stderr, "ERROR: overflow cluster offset=0x%" PRIx64
+                    "\n", cluster_offset);
+                errors++;
+            }
+        }
+    }
+
+    return errors;
+}
+
+/*
+ * Increases the refcount in the given refcount table for the all clusters
+ * referenced in the L2 table. While doing so, performs some checks on L2
+ * entries.
+ *
+ * Returns the number of errors found by the checks or -errno if an internal
+ * error occurred.
+ */
+static int check_refcounts_l2(BlockDriverState *bs,
+    uint16_t *refcount_table, int refcount_table_size, int64_t l2_offset,
+    int check_copied)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l2_table, offset;
+    int i, l2_size, nb_csectors, refcount;
+    int errors = 0;
+
+    /* Read L2 table from disk */
+    l2_size = s->l2_size * sizeof(uint64_t);
+    l2_table = qemu_malloc(l2_size);
+
+    if (bdrv_pread(s->hd, l2_offset, l2_table, l2_size) != l2_size)
+        goto fail;
+
+    /* Do the actual checks */
+    for(i = 0; i < s->l2_size; i++) {
+        offset = be64_to_cpu(l2_table[i]);
+        if (offset != 0) {
+            if (offset & QCOW_OFLAG_COMPRESSED) {
+                /* Compressed clusters don't have QCOW_OFLAG_COPIED */
+                if (offset & QCOW_OFLAG_COPIED) {
+                    fprintf(stderr, "ERROR: cluster %" PRId64 ": "
+                        "copied flag must never be set for compressed "
+                        "clusters\n", offset >> s->cluster_bits);
+                    offset &= ~QCOW_OFLAG_COPIED;
+                    errors++;
+                }
+
+                /* Mark cluster as used */
+                nb_csectors = ((offset >> s->csize_shift) &
+                               s->csize_mask) + 1;
+                offset &= s->cluster_offset_mask;
+                errors += inc_refcounts(bs, refcount_table,
+                              refcount_table_size,
+                              offset & ~511, nb_csectors * 512);
+            } else {
+                /* QCOW_OFLAG_COPIED must be set iff refcount == 1 */
+                if (check_copied) {
+                    uint64_t entry = offset;
+                    offset &= ~QCOW_OFLAG_COPIED;
+                    refcount = get_refcount(bs, offset >> s->cluster_bits);
+                    if ((refcount == 1) != ((entry & QCOW_OFLAG_COPIED) != 0)) {
+                        fprintf(stderr, "ERROR OFLAG_COPIED: offset=%"
+                            PRIx64 " refcount=%d\n", entry, refcount);
+                        errors++;
+                    }
+                }
+
+                /* Mark cluster as used */
+                offset &= ~QCOW_OFLAG_COPIED;
+                errors += inc_refcounts(bs, refcount_table,
+                              refcount_table_size,
+                              offset, s->cluster_size);
+
+                /* Correct offsets are cluster aligned */
+                if (offset & (s->cluster_size - 1)) {
+                    fprintf(stderr, "ERROR offset=%" PRIx64 ": Cluster is not "
+                        "properly aligned; L2 entry corrupted.\n", offset);
+                    errors++;
+                }
+            }
+        }
+    }
+
+    qemu_free(l2_table);
+    return errors;
+
+fail:
+    fprintf(stderr, "ERROR: I/O error in check_refcounts_l1\n");
+    qemu_free(l2_table);
+    return -EIO;
+}
+
+/*
+ * Increases the refcount for the L1 table, its L2 tables and all referenced
+ * clusters in the given refcount table. While doing so, performs some checks
+ * on L1 and L2 entries.
+ *
+ * Returns the number of errors found by the checks or -errno if an internal
+ * error occurred.
+ */
+static int check_refcounts_l1(BlockDriverState *bs,
+                              uint16_t *refcount_table,
+                              int refcount_table_size,
+                              int64_t l1_table_offset, int l1_size,
+                              int check_copied)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table, l2_offset, l1_size2;
+    int i, refcount, ret;
+    int errors = 0;
+
+    l1_size2 = l1_size * sizeof(uint64_t);
+
+    /* Mark L1 table as used */
+    errors += inc_refcounts(bs, refcount_table, refcount_table_size,
+                  l1_table_offset, l1_size2);
+
+    /* Read L1 table entries from disk */
+    l1_table = qemu_malloc(l1_size2);
+    if (bdrv_pread(s->hd, l1_table_offset,
+                   l1_table, l1_size2) != l1_size2)
+        goto fail;
+    for(i = 0;i < l1_size; i++)
+        be64_to_cpus(&l1_table[i]);
+
+    /* Do the actual checks */
+    for(i = 0; i < l1_size; i++) {
+        l2_offset = l1_table[i];
+        if (l2_offset) {
+            /* QCOW_OFLAG_COPIED must be set iff refcount == 1 */
+            if (check_copied) {
+                refcount = get_refcount(bs, (l2_offset & ~QCOW_OFLAG_COPIED)
+                    >> s->cluster_bits);
+                if ((refcount == 1) != ((l2_offset & QCOW_OFLAG_COPIED) != 0)) {
+                    fprintf(stderr, "ERROR OFLAG_COPIED: l2_offset=%" PRIx64
+                        " refcount=%d\n", l2_offset, refcount);
+                    errors++;
+                }
+            }
+
+            /* Mark L2 table as used */
+            l2_offset &= ~QCOW_OFLAG_COPIED;
+            errors += inc_refcounts(bs, refcount_table,
+                          refcount_table_size,
+                          l2_offset,
+                          s->cluster_size);
+
+            /* L2 tables are cluster aligned */
+            if (l2_offset & (s->cluster_size - 1)) {
+                fprintf(stderr, "ERROR l2_offset=%" PRIx64 ": Table is not "
+                    "cluster aligned; L1 entry corrupted\n", l2_offset);
+                errors++;
+            }
+
+            /* Process and check L2 entries */
+            ret = check_refcounts_l2(bs, refcount_table, refcount_table_size,
+                l2_offset, check_copied);
+            if (ret < 0) {
+                goto fail;
+            }
+            errors += ret;
+        }
+    }
+    qemu_free(l1_table);
+    return errors;
+
+fail:
+    fprintf(stderr, "ERROR: I/O error in check_refcounts_l1\n");
+    qemu_free(l1_table);
+    return -EIO;
+}
+
+/*
+ * Checks an image for refcount consistency.
+ *
+ * Returns 0 if no errors are found, the number of errors in case the image is
+ * detected as corrupted, and -errno when an internal error occured.
+ */
+int check_refcounts(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t size;
+    int nb_clusters, refcount1, refcount2, i;
+    QCowSnapshot *sn;
+    uint16_t *refcount_table;
+    int ret, errors = 0;
+
+    size = bdrv_getlength(s->hd);
+    nb_clusters = size_to_clusters(s, size);
+    refcount_table = qemu_mallocz(nb_clusters * sizeof(uint16_t));
+
+    /* header */
+    errors += inc_refcounts(bs, refcount_table, nb_clusters,
+                  0, s->cluster_size);
+
+    /* current L1 table */
+    ret = check_refcounts_l1(bs, refcount_table, nb_clusters,
+                       s->l1_table_offset, s->l1_size, 1);
+    if (ret < 0) {
+        return ret;
+    }
+    errors += ret;
+
+    /* snapshots */
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn = s->snapshots + i;
+        check_refcounts_l1(bs, refcount_table, nb_clusters,
+                           sn->l1_table_offset, sn->l1_size, 0);
+    }
+    errors += inc_refcounts(bs, refcount_table, nb_clusters,
+                  s->snapshots_offset, s->snapshots_size);
+
+    /* refcount data */
+    errors += inc_refcounts(bs, refcount_table, nb_clusters,
+                  s->refcount_table_offset,
+                  s->refcount_table_size * sizeof(uint64_t));
+    for(i = 0; i < s->refcount_table_size; i++) {
+        int64_t offset;
+        offset = s->refcount_table[i];
+        if (offset != 0) {
+            errors += inc_refcounts(bs, refcount_table, nb_clusters,
+                          offset, s->cluster_size);
+        }
+    }
+
+    /* compare ref counts */
+    for(i = 0; i < nb_clusters; i++) {
+        refcount1 = get_refcount(bs, i);
+        refcount2 = refcount_table[i];
+        if (refcount1 != refcount2) {
+            fprintf(stderr, "ERROR cluster %d refcount=%d reference=%d\n",
+                   i, refcount1, refcount2);
+            errors++;
+        }
+    }
+
+    qemu_free(refcount_table);
+
+    return errors;
+}
+