On Tue, Mar 29, 2022 at 11:19:16AM +0200, Hanna Reitz wrote:
When rebuilding the refcount structures (when qemu-img check -r found
errors with refcount = 0, but reference count > 0), the new refcount
table defaults to being put at the image file end[1]. There is no good
reason for that except that it means we will not have to rewrite any
refblocks we already wrote to disk.
Changing the code to rewrite those refblocks is not too difficult,
though, so let us do that. That is beneficial for images on block
devices, where we cannot really write beyond the end of the image file.
Use this opportunity to add extensive comments to the code, and refactor
it a bit, getting rid of the backwards-jumping goto.
[1] Unless there is something allocated in the area pointed to by the
last refblock, so we have to write that refblock. In that case, we
try to put the reftable in there.
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1519071
Closes: https://gitlab.com/qemu-project/qemu/-/issues/941
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
block/qcow2-refcount.c | 332 +++++++++++++++++++++++++++++------------
1 file changed, 235 insertions(+), 97 deletions(-)
+static int rebuild_refcounts_write_refblocks(
+ BlockDriverState *bs, void **refcount_table, int64_t *nb_clusters,
+ int64_t first_cluster, int64_t end_cluster,
+ uint64_t **on_disk_reftable_ptr, uint32_t *on_disk_reftable_entries_ptr
+ )
As you are rewriting this into a helper function, should this function
take Error **errp,...
+ /* 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));
+ return refblock_offset;
...instead of using fprintf(stderr), where the caller then handles the
error by printing?
Could be done as a separate patch.
+ /* Refblock is allocated, write it to disk */
+
ret = qcow2_pre_write_overlap_check(bs, 0, refblock_offset,
s->cluster_size, false);
if (ret < 0) {
fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
- goto fail;
+ return ret;
}
Another spot where errp conversion might improve the code.
- /* The size of *refcount_table is always cluster-aligned, therefore the
- * write operation will not overflow */
+ /*
+ * The refblock is simply a slice of *refcount_table.
+ * Note that the size of *refcount_table is always aligned to
+ * whole clusters, so the write operation will not result in
+ * out-of-bounds accesses.
+ */
on_disk_refblock = (void *)((char *) *refcount_table +
refblock_index * s->cluster_size);
@@ -2550,23 +2579,99 @@ write_refblocks:
s->cluster_size);
if (ret < 0) {
fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
- goto fail;
+ return ret;
}
and another
- /* Go to the end of this refblock */
+ /* This refblock is done, skip to its end */
cluster = refblock_start + s->refcount_block_size - 1;
}
- if (reftable_offset < 0) {
- uint64_t post_refblock_start, reftable_clusters;
+ return reftable_grown;
+}
The helper function looks okay.
+
+/*
+ * Creates a new refcount structure based solely on the in-memory information
+ * given through *refcount_table (this in-memory information is basically just
+ * the concatenation of all refblocks). 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,
+ void **refcount_table,
+ int64_t *nb_clusters)
+{
+ BDRVQcow2State *s = bs->opaque;
+ int64_t reftable_offset = -1;
+ int64_t reftable_length = 0;
+ int64_t reftable_clusters;
+ int64_t refblock_index;
+ uint32_t on_disk_reftable_entries = 0;
+ uint64_t *on_disk_reftable = NULL;
+ int ret = 0;
+ int reftable_size_changed = 0;
+ struct {
+ uint64_t reftable_offset;
+ uint32_t reftable_clusters;
+ } QEMU_PACKED reftable_offset_and_clusters;
+
+ qcow2_cache_empty(bs, s->refcount_block_cache);
+
+ /*
+ * For each refblock containing entries, we try to allocate a
+ * cluster (in the in-memory refcount table) and write its offset
+ * into on_disk_reftable[]. We then write the whole refblock to
+ * disk (as a slice of the in-memory refcount table).
+ * This is done by rebuild_refcounts_write_refblocks().
+ *
+ * Once we have scanned all clusters, we try to find space for the
+ * reftable. This will dirty the in-memory refcount table (i.e.
+ * make it differ from the refblocks we have already written), so we
+ * need to run rebuild_refcounts_write_refblocks() again for the
+ * range of clusters where the reftable has been allocated.
+ *
+ * This second run might make the reftable grow again, in which case
+ * we will need to allocate another space for it, which is why we
+ * repeat all this until the reftable stops growing.
+ *
+ * (This loop will terminate, because with every cluster the
+ * reftable grows, it can accomodate a multitude of more refcounts,
+ * so that at some point this must be able to cover the reftable
+ * and all refblocks describing it.)
+ *
+ * We then convert the reftable to big-endian and write it to disk.
+ *
+ * Note that we never free any reftable allocations. Doing so would
+ * needlessly complicate the algorithm: The eventual second check
+ * run we do will clean up all leaks we have caused.
+ */
Freeing reftable allocations from the first run might allow the second
(or third) to find a spot earlier in the image that is large enough to
contain the resized reftable, compared to leaving it leaked and
forcing subsequent runs to look later into the image. But I agree
that the complication of code needed to handle that is not worth the
minor corner-case savings of a more densely packed overall image (the
leaked clusters will probably remain sparse for any decent cluster
size).
Another approach might be to intentionally over-allocate the reftable
to the point where we know it can't grow, then allocate, then scale it
back down to how much we actually used (possibly reclaiming a few
clusters at the end of the allocation). But that's an even bigger
rewrite, and again, not worth the headache.
512-byte clusters would be where this is most likely to be noticeable
(that is, hitting a 3rd iteration with 512-byte clusters is probably
easy enough to actually test, but hitting a 3rd iteration with 2M
clusters is probably prohibitively expensive if even possible).
+
+ reftable_size_changed =
+ rebuild_refcounts_write_refblocks(bs, refcount_table, nb_clusters,
+ 0, *nb_clusters,
+ &on_disk_reftable,
+ &on_disk_reftable_entries);
+ if (reftable_size_changed < 0) {
+ res->check_errors++;
+ ret = reftable_size_changed;
+ goto fail;
+ }
+
+ /*
+ * There was no reftable before, so rebuild_refcounts_write_refblocks()
+ * must have increased its size (from 0 to something).
+ */
+ assert(reftable_size_changed == true);
'int == bool'. Works, but is a bit odd. I might have done just
assert(reftable_size_changed), since we just ruled out negative values
above. Particularly since...
+
+ do {
+ int64_t reftable_start_cluster, reftable_end_cluster;
+ int64_t first_free_cluster = 0;
...
+
+ /*
+ * If the reftable size has changed, we will need to find a new
+ * allocation, repeating the loop.
+ */
+ } while (reftable_size_changed);
...here you ARE using the int as a bool directly.
Reviewed-by: Eric Blake <eblake@redhat.com>