Re: [Qemu-devel] [PATCH v6 08/11] qcow2: Rebuild refcount structure during check

[Kevin Wolf <kwolf@redhat.com>]

Am 20.10.2014 um 16:35 hat Max Reitz geschrieben:
> 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. This leak will be
> dealt with in a follow-up commit.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 296 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 293 insertions(+), 3 deletions(-)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 183fc5b..75e726b 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -1642,6 +1642,276 @@ static void compare_refcounts(BlockDriverState *bs, BdrvCheckResult *res,
>  }
>  
>  /*
> + * Allocates a cluster using an in-memory refcount table (IMRT) in contrast to

s/a cluster/clusters/

> + * the on-disk refcount structures.
> + *
> + * On input, *first_free_cluster tells where to start looking, and need not
> + * actually be a free cluster; the returned offset will not be before that
> + * cluster.  On output, *first_free_cluster points to the first gap found, even
> + * if that gap was too small to be used as the returned offset.
> + *
> + * 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,

Having cluster_count and nb_clusters at the same time is confusing.

Maybe imrt_nb_clusters for this one?

> +                                   int64_t *first_free_cluster)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int64_t cluster = *first_free_cluster, i;
> +    bool first_gap = true;
> +    int contiguous_free_clusters;
> +
> +    /* Starting at *first_free_cluster, find a range of at least cluster_count
> +     * continuously free clusters */
> +    for (contiguous_free_clusters = 0;
> +         cluster < *nb_clusters && contiguous_free_clusters < cluster_count;
> +         cluster++)
> +    {
> +        if (!(*refcount_table)[cluster]) {
> +            contiguous_free_clusters++;
> +            if (first_gap) {
> +                /* If this is the first free cluster found, update
> +                 * *first_free_cluster accordingly */
> +                *first_free_cluster = cluster;
> +                first_gap = false;
> +            }
> +        } else if (contiguous_free_clusters) {
> +            contiguous_free_clusters = 0;
> +        }
> +    }
> +
> +    /* If contiguous_free_clusters is greater than zero, it contains the number
> +     * of continuously free clusters until the current cluster; the first free
> +     * cluster in the current "gap" is therefore
> +     * cluster - contiguous_free_clusters */
> +
> +    /* If no such range could be found, grow the in-memory refcount table
> +     * accordingly to append free clusters at the end of the image */
> +    if (contiguous_free_clusters < cluster_count) {
> +        int64_t old_nb_clusters = *nb_clusters;
> +
> +        /* contiguous_free_clusters clusters are already empty at the image end;
> +         * we need cluster_count clusters; therefore, we have to allocate
> +         * cluster_count - contiguous_free_clusters new clusters at the end of
> +         * the image (which is the current value of cluster; note that cluster
> +         * may exceed old_nb_clusters if *first_free_cluster pointed beyond the
> +         * image end) */
> +        *nb_clusters = cluster + cluster_count - contiguous_free_clusters;
> +        *refcount_table = g_try_realloc(*refcount_table,
> +                                        *nb_clusters * sizeof(uint16_t));
> +        if (!*refcount_table) {
> +            return -ENOMEM;

This means that on failure the passed refcount table pointer may be
overwritten by NULL. This is a surprising interface and should at least
be mentioned in the function comment.

But as the original IMRT is leaked here, too, it's probably better to
change the interface to leave *refcount_table alone if the failure case.

> +        }
> +
> +        memset(*refcount_table + old_nb_clusters, 0,
> +               (*nb_clusters - old_nb_clusters) * sizeof(uint16_t));
> +    }
> +
> +    /* Go back to the first free cluster */
> +    cluster -= contiguous_free_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, reftable_offset = -1, cluster = 0;
> +    int64_t refblock_offset, refblock_start, refblock_index;
> +    uint32_t reftable_size = 0;
> +    uint64_t *reftable = NULL;

refcount_table and reftable? Seriously?

> +    uint16_t *on_disk_refblock;
> +    int i, ret = 0;
> +    struct {
> +        uint64_t reftable_offset;
> +        uint32_t reftable_clusters;
> +    } QEMU_PACKED reftable_offset_and_clusters;
> +
> +    qcow2_cache_empty(bs, s->refcount_block_cache);
> +
> +write_refblocks:
> +    for (; cluster < *nb_clusters; cluster++) {
> +        if (!(*refcount_table)[cluster]) {
> +            continue;
> +        }
> +
> +        refblock_index = cluster >> s->refcount_block_bits;
> +        refblock_start = refblock_index << s->refcount_block_bits;
> +
> +        /* Don't allocate a cluster in a refblock already written to disk */
> +        if (first_free_cluster < refblock_start) {
> +            first_free_cluster = refblock_start;
> +        }
> +        refblock_offset = alloc_clusters_imrt(bs, 1, refcount_table,
> +                                              nb_clusters, &first_free_cluster);
> +        if (refblock_offset < 0) {
> +            fprintf(stderr, "ERROR allocating refblock: %s\n",
> +                    strerror(-refblock_offset));
> +            res->check_errors++;
> +            ret = refblock_offset;
> +            goto fail;
> +        }
> +
> +        if (reftable_size <= refblock_index) {
> +            uint32_t old_rt_size = reftable_size;
> +            reftable_size = ROUND_UP((refblock_index + 1) * sizeof(uint64_t),
> +                                     s->cluster_size) / sizeof(uint64_t);
> +            reftable = g_try_realloc(reftable,
> +                                     reftable_size * sizeof(uint64_t));

Another leak here.

> +            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 */
> +            reftable_offset = -1;
> +        }
> +        reftable[refblock_index] = refblock_offset;
> +
> +        /* If this is apparently the last refblock (for now), try to squeeze the
> +         * reftable in */
> +        if (refblock_index == (*nb_clusters - 1) >> s->refcount_block_bits &&
> +            reftable_offset < 0)
> +        {
> +            uint64_t reftable_clusters = size_to_clusters(s, reftable_size *
> +                                                          sizeof(uint64_t));
> +            reftable_offset = alloc_clusters_imrt(bs, reftable_clusters,
> +                                                  refcount_table, nb_clusters,
> +                                                  &first_free_cluster);
> +            if (reftable_offset < 0) {
> +                fprintf(stderr, "ERROR allocating reftable: %s\n",
> +                        strerror(-reftable_offset));
> +                res->check_errors++;
> +                ret = reftable_offset;
> +                goto fail;
> +            }
> +        }
> +
> +        ret = qcow2_pre_write_overlap_check(bs, 0, refblock_offset,
> +                                            s->cluster_size);
> +        if (ret < 0) {
> +            fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
> +            goto fail;
> +        }
> +
> +        on_disk_refblock = g_malloc0(s->cluster_size);

qemu_blockalign?

> +        for (i = 0; i < s->cluster_size / sizeof(uint16_t) &&
> +                    refblock_start + i < *nb_clusters; i++)
> +        {
> +            on_disk_refblock[i] =
> +                cpu_to_be16((*refcount_table)[refblock_start + i]);
> +        }
> +
> +        ret = bdrv_write(bs->file, refblock_offset / BDRV_SECTOR_SIZE,
> +                         (void *)on_disk_refblock, s->cluster_sectors);
> +        g_free(on_disk_refblock);
> +        if (ret < 0) {
> +            fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
> +            goto fail;
> +        }
> +
> +        /* Go to the end of this refblock */
> +        cluster = refblock_start + s->cluster_size / sizeof(uint16_t) - 1;
> +    }
> +
> +    if (reftable_offset < 0) {
> +        uint64_t post_refblock_start, reftable_clusters;
> +
> +        post_refblock_start = ROUND_UP(*nb_clusters,
> +                                       s->cluster_size / sizeof(uint16_t));
> +        reftable_clusters = size_to_clusters(s,
> +                                             reftable_size * sizeof(uint64_t));
> +        /* Not pretty but simple */
> +        if (first_free_cluster < post_refblock_start) {
> +            first_free_cluster = post_refblock_start;
> +        }
> +        reftable_offset = alloc_clusters_imrt(bs, reftable_clusters,
> +                                              refcount_table, nb_clusters,
> +                                              &first_free_cluster);
> +        if (reftable_offset < 0) {
> +            fprintf(stderr, "ERROR allocating reftable: %s\n",
> +                    strerror(-reftable_offset));
> +            res->check_errors++;
> +            ret = reftable_offset;
> +            goto fail;
> +        }
> +
> +        goto write_refblocks;
> +    }

Ouch. :-)

Well, it should work.

> +    assert(reftable);
> +
> +    for (refblock_index = 0; refblock_index < reftable_size; refblock_index++) {
> +        cpu_to_be64s(&reftable[refblock_index]);
> +    }
> +
> +    ret = qcow2_pre_write_overlap_check(bs, 0, reftable_offset,
> +                                        reftable_size * sizeof(uint64_t));
> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR writing reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    ret = bdrv_write(bs->file, reftable_offset / BDRV_SECTOR_SIZE,
> +                     (void *)reftable,
> +                     reftable_size * sizeof(uint64_t) / BDRV_SECTOR_SIZE);

Why not bdrv_pwrite when you only have byte offset and length?

> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR writing reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    /* Enter new reftable into the image header */
> +    cpu_to_be64w(&reftable_offset_and_clusters.reftable_offset,
> +                 reftable_offset);
> +    cpu_to_be32w(&reftable_offset_and_clusters.reftable_clusters,
> +                 size_to_clusters(s, reftable_size * sizeof(uint64_t)));
> +    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader,
> +                                              refcount_table_offset),
> +                           &reftable_offset_and_clusters,
> +                           sizeof(reftable_offset_and_clusters));
> +    if (ret < 0) {
> +        fprintf(stderr, "ERROR setting reftable: %s\n", strerror(-ret));
> +        goto fail;
> +    }
> +
> +    for (refblock_index = 0; refblock_index < reftable_size; refblock_index++) {
> +        be64_to_cpus(&reftable[refblock_index]);
> +    }
> +    s->refcount_table = reftable;
> +    s->refcount_table_offset = reftable_offset;
> +    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
> @@ -1651,6 +1921,7 @@ 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;
> @@ -1677,11 +1948,30 @@ 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");

Is it safe in this case to continue with fixing leaks? Should some error
counter be increased?

> +        }
> +
> +        if (res->leaks || res->corruptions) {
> +            *res = pre_compare_res;
> +            compare_refcounts(bs, res, fix, &rebuild, &highest_cluster,
> +                              refcount_table, nb_clusters);
> +        }
>      }

Kevin