Re: [Qemu-devel] [PATCH 2/3] qcow2: Implement bdrv_amend_options

[Kevin Wolf <kwolf@redhat.com>]

Am 28.08.2013 um 10:08 hat Max Reitz geschrieben:
> Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
> and lazy_refcounts.
> 
> Downgrading images from compat=1.1 to compat=0.10 is achieved through
> handling all incompatible flags accordingly, clearing all compatible and
> autoclear flags and expanding all zero clusters.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cluster.c |  66 ++++++++++++++++++
>  block/qcow2.c         | 182 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2.h         |   2 +
>  3 files changed, 250 insertions(+)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index cca76d4..ac50db2 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1476,3 +1476,69 @@ fail:
>  
>      return ret;
>  }
> +
> +/*
> + * Expands all zero clusters on the image; important for downgrading to a qcow2
> + * version which doesn't yet support metadata zero clusters.
> + */
> +int qcow2_expand_zero_clusters(BlockDriverState *bs)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int ret;
> +    int i;
> +
> +    for (i = 0; i < s->l1_size; i++) {

This fails to expand zero clusters in non-active L2 tables. (Please add
a test case for this scenario.)

> +        uint64_t *l2_table;
> +        int l2_index;
> +        int j;
> +        bool l2_dirty = false;
> +
> +        ret = get_cluster_table(bs, (uint64_t)i << (s->l2_bits +
> +                s->cluster_bits), &l2_table, &l2_index);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +
> +        for (j = 0; j < s->l2_size; j++) {
> +            uint64_t l2_entry = be64_to_cpu(l2_table[j]);
> +            if (!(l2_entry & QCOW_OFLAG_COMPRESSED) &&
> +                (l2_entry & QCOW_OFLAG_ZERO)) {

qcow2_get_cluster_type()?

> +                /* uncompressed zero cluster */
> +                int64_t offset = qcow2_alloc_clusters(bs, s->cluster_size);
> +                if (offset < 0) {
> +                    ret = offset;
> +                    goto fail;
> +                }

Does it handle zero clusters with an offset (i.e. preallocation)
correctly? I believe we must either reuse that cluster or free it.

> +                ret = bdrv_write_zeroes(bs->file, offset >> BDRV_SECTOR_BITS,
> +                                        s->cluster_size >> BDRV_SECTOR_BITS);
> +                if (ret < 0) {
> +                    qcow2_free_clusters(bs, offset, s->cluster_size,
> +                            QCOW2_DISCARD_ALWAYS);
> +                    goto fail;
> +                }
> +
> +                l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
> +                l2_dirty = true;
> +            }
> +        }
> +
> +        ret = 0;
> +
> +fail:
> +        if (l2_dirty) {
> +            qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);

qcow2_cache_depends_on_flush(s->l2_table_cache), too. The L2 table must
only be written when the zeroes are stable on disk.

> +        }
> +
> +        if (ret < 0) {
> +            qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
> +        } else {
> +            ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
> +        }
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    return 0;
> +}
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 78097e5..47cd5ad 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1735,6 +1735,187 @@ static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
>      return ret;
>  }
>  
> +/*
> + * Downgrades an image's version. To achieve this, any incompatible features
> + * have to be removed.
> + */
> +static int qcow2_downgrade(BlockDriverState *bs, int target_version)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int current_version = s->qcow_version;
> +    int ret;
> +
> +    if (target_version == current_version) {
> +        return 0;
> +    } else if (target_version > current_version) {
> +        return -EINVAL;
> +    } else if (target_version != 2) {
> +        return -EINVAL;
> +    }
> +
> +    /* clear incompatible features */
> +    if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
> +        BdrvCheckResult result;
> +        ret = qcow2_check(bs, &result, BDRV_FIX_ERRORS);
> +        if (ret < 0) {
> +            return ret;
> +        }

This is unnecessary: The image could be opened, so we know that it was
clean when we started. We also know that we haven't crashed yet, so if we
flush all in-memory data, we'll have a consistent on-disk state again.

qcow2_mark_clean() already calls bdrv_flush(bs), so it does everything
that is needed in this respect.

> +        qcow2_mark_clean(bs);

However, it can return errors, for which we should check.

> +    }
> +
> +    if (s->incompatible_features) {
> +        return -ENOTSUP;
> +    }
> +
> +    /* since we can ignore compatible features, we can set them to 0 as well */
> +    s->compatible_features = 0;
> +    /* if lazy refcounts have been used, they have already been fixed through
> +     * clearing the dirty flag */
> +
> +    /* clearing autoclear features is trivial */
> +    s->autoclear_features = 0;
> +
> +    /* the refcount order might be different in newer images - however, qemu
> +     * doesn't support anything different than 4 anyway, so nothing to fix
> +     * there */
> +
> +    ret = qcow2_expand_zero_clusters(bs);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    s->qcow_version = target_version;
> +    ret = qcow2_update_header(bs);
> +    if (ret < 0) {
> +        s->qcow_version = current_version;
> +        return ret;
> +    }
> +    return 0;
> +}
> +
> +static int qcow2_amend_options(BlockDriverState *bs,
> +                               QEMUOptionParameter *options)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int old_version = s->qcow_version, new_version = old_version;
> +    uint64_t new_size = 0;
> +    const char *backing_file = NULL, *backing_format = NULL;
> +    bool lazy_refcounts = s->use_lazy_refcounts;
> +    int ret;
> +    int i;
> +
> +    for (i = 0; options[i].name; i++)
> +    {
> +        if (!strcmp(options[i].name, "compat")) {
> +            if (!options[i].value.s) {
> +                /* preserve default */
> +            } else if (!strcmp(options[i].value.s, "0.10")) {
> +                new_version = 2;
> +            } else if (!strcmp(options[i].value.s, "1.1")) {
> +                new_version = 3;
> +            } else {
> +                fprintf(stderr, "Unknown compatibility level %s.\n",
> +                        options[i].value.s);
> +                return -EINVAL;
> +            }
> +        } else if (!strcmp(options[i].name, "preallocation")) {
> +            if (options[i].value.s) {
> +                fprintf(stderr, "Cannot change preallocation mode.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "size")) {
> +            new_size = options[i].value.n;
> +        } else if (!strcmp(options[i].name, "backing_file")) {
> +            backing_file = options[i].value.s;
> +        } else if (!strcmp(options[i].name, "backing_fmt")) {
> +            backing_format = options[i].value.s;
> +        } else if (!strcmp(options[i].name, "encryption")) {
> +            if (options[i].value.n != !!s->crypt_method) {
> +                fprintf(stderr, "Changing the encryption flag is not "
> +                        "supported.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "cluster_size")) {
> +            if (options[i].value.n && (options[i].value.n != s->cluster_size)) {
> +                fprintf(stderr, "Changing the cluster size is not "
> +                        "supported.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
> +            /* TODO: detect whether this flag was indeed explicitly given */
> +            lazy_refcounts = options[i].value.n;

I can see two ways to achieve this:

1. Add a new field 'bool assigned' to QEMUOptionParameter, which would
   be cleared before parsing an option string and set for each option in
   set_option_parameter()

2. Get the QemuOpts conversion series in and add a function that tells
   whether a given option was specified or not.

The same TODO should actually apply to encryption and cluster_size as
well, shouldn't it?

> +        } else {
> +            fprintf(stderr, "Unknown option '%s'.\n", options[i].name);

That's actually a programming error, perhaps a case for assert(false);

> +        }
> +    }
> +
> +    if (new_version != old_version) {
> +        if (new_version > old_version) {
> +            /* Upgrade */
> +            s->qcow_version = new_version;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->qcow_version = old_version;
> +                return ret;
> +            }
> +        } else {
> +            qcow2_downgrade(bs, new_version);

Error handling?

> +        }
> +    }
> +
> +    if (new_size) {
> +        ret = qcow2_truncate(bs, new_size);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    if (backing_file || backing_format) {
> +        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
> +                                        backing_format ?: bs->backing_format);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    if (s->use_lazy_refcounts != lazy_refcounts) {
> +        if (lazy_refcounts) {
> +            if (s->qcow_version < 3) {
> +                fprintf(stderr, "Lazy refcounts only supported with compatibility "
> +                        "level 1.1 and above (use compat=1.1 or greater)\n");
> +                return -EINVAL;
> +            }
> +            s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
> +                return ret;
> +            }
> +            s->use_lazy_refcounts = true;
> +        } else {
> +            /* make image clean first */
> +            if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
> +                BdrvCheckResult result;
> +                ret = qcow2_check(bs, &result, BDRV_FIX_ERRORS);
> +                if (ret < 0) {
> +                    return ret;
> +                }
> +            }

Unnecessary, like above.

> +            qcow2_mark_clean(bs);

And error handling again.

> +            /* now disallow lazy refcounts */
> +            s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
> +                return ret;
> +            }
> +            s->use_lazy_refcounts = false;
> +        }
> +    }
> +
> +    return 0;
> +}

Kevin