Re: [Qemu-devel] [PATCH v3 07/10] qcow2: Rebuild refcount structure during check

["Benoît Canet" <benoit.canet@nodalink.com>]

On Fri, Aug 22, 2014 at 06:31:41PM +0200, Max Reitz wrote:
> The previous commit introduced the "rebuild" variable to qcow2's
> implementation of the image consistency check. Now make use of this by
> adding a function which creates a completely new refcount structure
> based solely on the in-memory information gathered before.
> 
> The old refcount structure will be leaked, however.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 265 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 262 insertions(+), 3 deletions(-)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 242a20c..59cab65 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -1591,6 +1591,245 @@ static void compare_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>  }
>  
>  /*
> + * Allocates a cluster using an in-memory refcount table (IMRT) in contrast to
> + * the on-disk refcount structures.
> + *
> + * *first_free_cluster does not necessarily point to the first free cluster, but
> + * may point to one cluster as close as possible before it. The offset returned
> + * will never be before that cluster.
> + *
> + * Note that *first_free_cluster is a cluster index whereas the return value is
> + * an offset.
> + */
> +static int64_t alloc_clusters_imrt(BlockDriverState *bs,
> +                                   int cluster_count,
> +                                   uint16_t **refcount_table,
> +                                   int64_t *nb_clusters,
> +                                   int64_t *first_free_cluster)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int64_t cluster = *first_free_cluster, i;
> +    bool first_gap = true;
> +    int contiguous_clusters;
> +


> +    for (contiguous_clusters = 0;
> +         cluster < *nb_clusters && contiguous_clusters < cluster_count;
> +         cluster++)
> +    {
> +        if (!(*refcount_table)[cluster]) {
> +            contiguous_clusters++;
> +            if (first_gap) {
> +                *first_free_cluster = cluster;
> +                first_gap = false;
> +            }
> +        } else if (contiguous_clusters) {
> +            contiguous_clusters = 0;
> +        }
> +    }

s/contignuous_clusters/contiguous_free_clusters/

I think I understood the above block better than the first time but I it would
worth a comment on it's rather complex purpose.

> +
> +    if (contiguous_clusters < cluster_count) {
> +        int64_t old_nb_clusters = *nb_clusters;
> +
> +        *nb_clusters = cluster + cluster_count - contiguous_clusters;
> +        *refcount_table = g_try_realloc(*refcount_table,
> +                                        *nb_clusters * sizeof(uint16_t));
> +        if (!*refcount_table) {
> +            return -ENOMEM;
> +        }
> +
> +        memset(*refcount_table + old_nb_clusters, 0,
> +               (*nb_clusters - old_nb_clusters) * sizeof(uint16_t));
> +    }
> +
> +    cluster -= contiguous_clusters;
> +    for (i = 0; i < cluster_count; i++) {
> +        (*refcount_table)[cluster + i] = 1;
> +    }
> +
> +    return cluster << s->cluster_bits;
> +}
> +
> +/*
> + * Creates a new refcount structure based solely on the in-memory information
> + * given through *refcount_table. All necessary allocations will be reflected
> + * in that array.
> + *
> + * On success, the old refcount structure is leaked (it will be covered by the
> + * new refcount structure).
> + */
> +static int rebuild_refcount_structure(BlockDriverState *bs,
> +                                      BdrvCheckResult *res,
> +                                      uint16_t **refcount_table,
> +                                      int64_t *nb_clusters)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int64_t first_free_cluster = 0, rt_ofs = -1, cluster = 0;

/* rb stands for refcount block and rt stands for refcount table */

> +    int64_t rb_ofs, rb_start, rb_index;
> +    uint32_t reftable_size = 0;
> +    uint64_t *reftable = NULL;
> +    uint16_t *on_disk_rb;
> +    uint8_t rt_offset_and_clusters[sizeof(uint64_t) + sizeof(uint32_t)];
> +    int i, ret = 0;
> +
> +    qcow2_cache_empty(bs, s->refcount_block_cache);
> +
> +write_refblocks:
> +    for (; cluster < *nb_clusters; cluster++) {
> +        if (!(*refcount_table)[cluster]) {
> +            continue;
> +        }
> +
> +        rb_index = cluster >> (s->cluster_bits - 1);
> +        rb_start = rb_index << (s->cluster_bits - 1);
> +
> +        /* Don't allocate a cluster in a refblock already written to disk */
> +        if (first_free_cluster < rb_start) {
> +            first_free_cluster = rb_start;
> +        }
> +        rb_ofs = alloc_clusters_imrt(bs, 1, refcount_table, nb_clusters,
> +                                     &first_free_cluster);
> +        if (rb_ofs < 0) {
> +            fprintf(stderr, "ERROR allocating refblock: %s\n", strerror(-ret));
> +            res->check_errors++;
> +            ret = rb_ofs;
> +            goto fail;
> +        }
> +
> +        if (reftable_size <= rb_index) {
> +            uint32_t old_rt_size = reftable_size;
> +            reftable_size = ROUND_UP((rb_index + 1) * sizeof(uint64_t),
> +                                     s->cluster_size) / sizeof(uint64_t);
> +            reftable = g_try_realloc(reftable,
> +                                     reftable_size * sizeof(uint64_t));
> +            if (!reftable) {
> +                res->check_errors++;
> +                ret = -ENOMEM;
> +                goto fail;
> +            }
> +
> +            memset(reftable + old_rt_size, 0,
> +                   (reftable_size - old_rt_size) * sizeof(uint64_t));
> +
> +            /* The offset we have for the reftable is now no longer valid;
> +             * this will leak that range, but we can easily fix that by running
> +             * a leak-fixing check after this rebuild operation */
> +            rt_ofs = -1;
> +        }
> +        reftable[rb_index] = rb_ofs;
> +
> +        /* If this is apparently the last refblock (for now), try to squeeze the
> +         * reftable in */
> +        if (rb_index == (*nb_clusters - 1) >> (s->cluster_bits - 1)
                                                                    ^ I don't understand the - 1 here.
> &&
> +            rt_ofs < 0)
> +        {
> +            rt_ofs = alloc_clusters_imrt(bs, size_to_clusters(s, reftable_size *
> +                                                              sizeof(uint64_t)),
> +                                         refcount_table, nb_clusters,
> +                                         &first_free_cluster);
> +            if (rt_ofs < 0) {
> +                fprintf(stderr, "ERROR allocating reftable: %s\n",
> +                        strerror(-ret));
> +                res->check_errors++;
> +                ret = rt_ofs;
> +                goto fail;
> +            }
> +        }
> +
> +        ret = qcow2_pre_write_overlap_check(bs, 0, rb_ofs, s->cluster_size);
> +        if (ret < 0) {
> +            fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
> +            goto fail;
> +        }
> +
> +        on_disk_rb = g_malloc0(s->cluster_size);
> +        for (i = 0; i < s->cluster_size / sizeof(uint16_t) &&
> +                    rb_start + i < *nb_clusters; i++)
> +        {
> +            on_disk_rb[i] = cpu_to_be16((*refcount_table)[rb_start + i]);
> +        }
> +
> +        ret = bdrv_write(bs->file, rb_ofs / BDRV_SECTOR_SIZE,
> +                         (void *)on_disk_rb, s->cluster_sectors);
> +        g_free(on_disk_rb);
> +        if (ret < 0) {
> +            fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
> +            goto fail;
> +        }
> +
> +        /* Go to the end of this refblock */
> +        cluster = rb_start + s->cluster_size / sizeof(uint16_t) - 1;
> +    }
> +
> +    if (rt_ofs < 0) {
> +        int64_t post_rb_start = ROUND_UP(*nb_clusters,
> +                                         s->cluster_size / sizeof(uint16_t));
> +
> +        /* Not pretty but simple */
> +        if (first_free_cluster < post_rb_start) {
> +            first_free_cluster = post_rb_start;
> +        }
> +        rt_ofs = alloc_clusters_imrt(bs, size_to_clusters(s, reftable_size *
> +                                                          sizeof(uint64_t)),
> +                                     refcount_table, nb_clusters,
> +                                     &first_free_cluster);
> +        if (rt_ofs < 0) {
> +            fprintf(stderr, "ERROR allocating reftable: %s\n", strerror(-ret));
> +            res->check_errors++;
> +            ret = rt_ofs;
> +            goto fail;
> +        }
> +
> +        goto write_refblocks;
> +    }
> +
> +    assert(reftable);
> +
> +    for (rb_index = 0; rb_index < reftable_size; rb_index++) {
> +        cpu_to_be64s(&reftable[rb_index]);
> +    }
> +
> +    ret = qcow2_pre_write_overlap_check(bs, 0, rt_ofs,
> +                                        reftable_size * sizeof(uint64_t));
> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR writing reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    ret = bdrv_write(bs->file, rt_ofs / BDRV_SECTOR_SIZE, (void *)reftable,
> +                     reftable_size * sizeof(uint64_t) / BDRV_SECTOR_SIZE);
> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR writing reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    /* Enter new reftable into the image header */
> +    cpu_to_be64w((uint64_t *)&rt_offset_and_clusters[0], rt_ofs);
> +    cpu_to_be32w((uint32_t *)&rt_offset_and_clusters[sizeof(uint64_t)],
> +                 size_to_clusters(s, reftable_size * sizeof(uint64_t)));
> +    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader,
> +                                              refcount_table_offset),
> +                           rt_offset_and_clusters,
> +                           sizeof(rt_offset_and_clusters));
> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR setting reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    for (rb_index = 0; rb_index < reftable_size; rb_index++) {
> +        be64_to_cpus(&reftable[rb_index]);
> +    }
> +    s->refcount_table = reftable;
> +    s->refcount_table_offset = rt_ofs;
> +    s->refcount_table_size = reftable_size;
> +
> +    return 0;
> +
> +fail:
> +    g_free(reftable);
> +    return ret;
> +}
> +
> +/*
>   * Checks an image for refcount consistency.
>   *
>   * Returns 0 if no errors are found, the number of errors in case the image is
> @@ -1600,6 +1839,7 @@ int qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>                            BdrvCheckMode fix)
>  {
>      BDRVQcowState *s = bs->opaque;
> +    BdrvCheckResult pre_compare_res;
>      int64_t size, highest_cluster, nb_clusters;
>      uint16_t *refcount_table = NULL;
>      bool rebuild = false;
> @@ -1626,11 +1866,30 @@ int qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>          goto fail;
>      }
>  
> -    compare_refcounts(bs, res, fix, &rebuild, &highest_cluster, refcount_table,
> +    /* In case we don't need to rebuild the refcount structure (but want to fix
> +     * something), this function is immediately called again, in which case the
> +     * result should be ignored */
> +    pre_compare_res = *res;
> +    compare_refcounts(bs, res, 0, &rebuild, &highest_cluster, refcount_table,
>                        nb_clusters);
>  
> -    if (rebuild) {
> -        fprintf(stderr, "ERROR need to rebuild refcount structures\n");
> +    if (rebuild && (fix & BDRV_FIX_ERRORS)) {
> +        fprintf(stderr, "Rebuilding refcount structure\n");
> +        ret = rebuild_refcount_structure(bs, res, &refcount_table,
> +                                         &nb_clusters);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +    } else if (fix) {
> +        if (rebuild) {
> +            fprintf(stderr, "ERROR need to rebuild refcount structures\n");
> +        }
> +
> +        if (res->leaks || res->corruptions) {
> +            *res = pre_compare_res;
> +            compare_refcounts(bs, res, fix, &rebuild, &highest_cluster,
> +                              refcount_table, nb_clusters);
> +        }
>      }
>  
>      /* check OFLAG_COPIED */
> -- 
> 2.0.4
>