Re: [RFC PATCH v2 14/26] qcow2: Add subcluster support to qcow2_get_cluster_offset()

From: Max Reitz <mreitz@redhat.com>
To: Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
	Anton Nefedov <anton.nefedov@virtuozzo.com>,
	qemu-block@nongnu.org,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"Denis V . Lunev" <den@openvz.org>
Subject: Re: [RFC PATCH v2 14/26] qcow2: Add subcluster support to qcow2_get_cluster_offset()
Date: Mon, 4 Nov 2019 15:58:57 +0100	[thread overview]
Message-ID: <673d72da-bf8c-3ffb-a324-79e93f88a140@redhat.com> (raw)
In-Reply-To: <6932c2ddfe19a564cad7c54246290e166525fc46.1572125022.git.berto@igalia.com>

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On 26.10.19 23:25, Alberto Garcia wrote:
> The logic of this function remains pretty much the same, except that
> it uses count_contiguous_subclusters(), which combines the logic of
> count_contiguous_clusters() / count_contiguous_clusters_unallocated()
> and checks individual subclusters.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cluster.c | 111 ++++++++++++++++++++----------------------
>  1 file changed, 52 insertions(+), 59 deletions(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index 990bc070af..e67559152f 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -372,66 +372,51 @@ fail:
>  }
>  
>  /*
> - * Checks how many clusters in a given L2 slice are contiguous in the image
> - * file. As soon as one of the flags in the bitmask stop_flags changes compared
> - * to the first cluster, the search is stopped and the cluster is not counted
> - * as contiguous. (This allows it, for example, to stop at the first compressed
> - * cluster which may require a different handling)
> + * Return the number of contiguous subclusters of the exact same type
> + * in a given L2 slice, starting from cluster @l2_index, subcluster
> + * @sc_index. At most @nb_clusters are checked. Allocated clusters are

I’d add a note that reassures that @nb_clusters really is a number of
clusters, not subclusters.  (Because if the variable were named “x”
(i.e., it’d be “At most @x are checked”), you’d assume it refers to a
number of subclusters.)

OTOH, what I don’t like so far about this series is that the “cluster
logic” is still everywhere when I think it should just be about
subclusters now.  (Except in few places where it must be about clusters
as in something that can have a distinct host offset and/or has an own
L2 entry.)  So maybe the parameter should really be @nb_subclusters.

But I’m not sure.  For how this function is written right now, it makes
sense for it to be @nb_clusters, but I think it could be changed so it
would work with @nb_subclusters, too.  (Just a single loop over the
subclusters and then loading l2_entry+l2_bitmap and checking the offset
at every cluster boundary.)

> + * also required to be contiguous in the image file.
>   */
> -static int count_contiguous_clusters(BlockDriverState *bs, int nb_clusters,
> -        int cluster_size, uint64_t *l2_slice, int l2_index, uint64_t stop_flags)
> +static int count_contiguous_subclusters(BlockDriverState *bs, int nb_clusters,
> +                                        unsigned sc_index, uint64_t *l2_slice,
> +                                        int l2_index)
>  {
>      BDRVQcow2State *s = bs->opaque;
> -    int i;
> -    QCow2ClusterType first_cluster_type;
> -    uint64_t mask = stop_flags | L2E_OFFSET_MASK | QCOW_OFLAG_COMPRESSED;
> -    uint64_t first_entry = get_l2_entry(s, l2_slice, l2_index);
> -    uint64_t offset = first_entry & mask;
> -
> -    first_cluster_type = qcow2_get_cluster_type(bs, first_entry);
> -    if (first_cluster_type == QCOW2_CLUSTER_UNALLOCATED) {
> -        return 0;
> +    int i, j, count = 0;
> +    uint64_t l2_entry = get_l2_entry(s, l2_slice, l2_index);
> +    uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
> +    uint64_t expected_offset = l2_entry & L2E_OFFSET_MASK;
> +    bool check_offset = true;
> +    QCow2ClusterType type =
> +        qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
> +
> +    assert(type != QCOW2_CLUSTER_INVALID); /* The caller should check this */
> +
> +    if (type == QCOW2_CLUSTER_COMPRESSED) {
> +        return 1; /* Compressed clusters are always counted one by one */

Hm, yes, but cluster by cluster, not subcluster by subcluster, so this
should be s->subclusters_per_cluster, and perhaps sc_index should be
asserted to be 0.  (Or it should be s->subclusters_per_cluster - sc_index.)

[...]

> @@ -514,8 +499,8 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,

I suppose this is get_subcluster_offset now.

[...]

> @@ -587,6 +574,13 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
>          goto fail;
>      }
>      switch (type) {
> +    case QCOW2_CLUSTER_INVALID:
> +        qcow2_signal_corruption(bs, true, -1, -1, "Invalid cluster entry found "
> +                                " (L2 offset: %#" PRIx64 ", L2 index: %#x)",
> +                                l2_offset, l2_index);
> +        ret = -EIO;
> +        goto fail;
> +        break;

No need to break here.

>      case QCOW2_CLUSTER_COMPRESSED:
>          if (has_data_file(bs)) {
>              qcow2_signal_corruption(bs, true, -1, -1, "Compressed cluster "
> @@ -602,16 +596,15 @@ int qcow2_get_cluster_offset(BlockDriverState *bs, uint64_t offset,
>          break;

Here (QCOW2_CLUSTER_COMPRESSED), c is 1, even though it should count
subclusters.

Max

>      case QCOW2_CLUSTER_ZERO_PLAIN:
>      case QCOW2_CLUSTER_UNALLOCATED:
> -        /* how many empty clusters ? */
> -        c = count_contiguous_clusters_unallocated(bs, nb_clusters,
> -                                                  l2_slice, l2_index, type);
> +        c = count_contiguous_subclusters(bs, nb_clusters, sc_index,
> +                                         l2_slice, l2_index);
>          *cluster_offset = 0;
>          break;

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