qcow2: Document and enforce the QCowL2Meta invariants

The QCowL2Meta structure is used to store information about a part of
a write request that touches clusters that need changes in their L2
entries. This happens with newly-allocated clusters or subclusters.

This structure has changed a bit since it was first created and its
current documentation is not quite up-to-date.

A write request can span a region consisting of a combination of
clusters of different types, and qcow2_alloc_host_offset() can
repeatedly call handle_copied() and handle_alloc() to add more
clusters to the mix as long as they all are contiguous on the image
file.

Because of this a write request has a list of QCowL2Meta structures,
one for each part of the request that needs changes in the L2
metadata.

Each one of them spans nb_clusters and has two copy-on-write regions
located immediately before and after the middle region touched by that
part of the write request. Even when those regions themselves are
empty their offsets must be correct because they are used to know the
location of the middle region.

This was not always the case but it is not a problem anymore
because the only two places where QCowL2Meta structures are created
(calculate_l2_meta() and qcow2_co_truncate()) ensure that the
copy-on-write regions are correctly defined, and so do assertions like
the ones in perform_cow().

The conditional initialization of the 'written_to' variable is
therefore unnecessary and is removed by this patch.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201007161323.4667-1-berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 15 insertions, 4 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 4274806a2a..1b0733238b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2361,15 +2361,26 @@ static bool merge_cow(uint64_t offset, unsigned bytes,
             continue;
         }
 
-        /* The data (middle) region must be immediately after the
-         * start region */
+        /*
+         * The write request should start immediately after the first
+         * COW region. This does not always happen because the area
+         * touched by the request can be larger than the one defined
+         * by @m (a single request can span an area consisting of a
+         * mix of previously unallocated and allocated clusters, that
+         * is why @l2meta is a list).
+         */
         if (l2meta_cow_start(m) + m->cow_start.nb_bytes != offset) {
+            /* In this case the request starts before this region */
+            assert(offset < l2meta_cow_start(m));
+            assert(m->cow_start.nb_bytes == 0);
             continue;
         }
 
-        /* The end region must be immediately after the data (middle)
-         * region */
+        /* The write request should end immediately before the second
+         * COW region (see above for why it does not always happen) */
         if (m->offset + m->cow_end.offset != offset + bytes) {
+            assert(offset + bytes > m->offset + m->cow_end.offset);
+            assert(m->cow_end.nb_bytes == 0);
             continue;
         }