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qcow2: Fix preallocation on images with unaligned sizes

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When resizing an image with qcow2_co_truncate() using the falloc or
full preallocation modes the code assumes that both the old and new
sizes are cluster-aligned.

There are two problems with this:

  1) The calculation of how many clusters are involved does not always
     get the right result.

     Example: creating a 60KB image and resizing it (with
     preallocation=full) to 80KB won't allocate the second cluster.

  2) No copy-on-write is performed, so in the previous example if
     there is a backing file then the first 60KB of the first cluster
     won't be filled with data from the backing file.

This patch fixes both issues.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200617140036.20311-1-berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-06-17 16:00:36 +02:00 • committed by Max Reitz
parent e8de7ba9ea
commit a5675f3901
3 changed files with 47 additions and 3 deletions
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17
block/qcow2.c
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@ -4239,8 +4239,8 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
old_file_size = ROUND_UP(old_file_size, s->cluster_size);
}
nb_new_data_clusters = DIV_ROUND_UP(offset - old_length,
s->cluster_size);
nb_new_data_clusters = (ROUND_UP(offset, s->cluster_size) -
start_of_cluster(s, old_length)) >> s->cluster_bits;
/* This is an overestimation; we will not actually allocate space for
* these in the file but just make sure the new refcount structures are
@ -4317,10 +4317,21 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
int64_t nb_clusters = MIN(
nb_new_data_clusters,
s->l2_slice_size - offset_to_l2_slice_index(s, guest_offset));
QCowL2Meta allocation = {
unsigned cow_start_length = offset_into_cluster(s, guest_offset);
QCowL2Meta allocation;
guest_offset = start_of_cluster(s, guest_offset);
allocation = (QCowL2Meta) {
.offset = guest_offset,
.alloc_offset = host_offset,
.nb_clusters = nb_clusters,
.cow_start = {
.offset = 0,
.nb_bytes = cow_start_length,
},
.cow_end = {
.offset = nb_clusters << s->cluster_bits,
.nb_bytes = 0,
},
};
qemu_co_queue_init(&allocation.dependent_requests);