[v9,24/34] qcow2: Add subcluster support to check_refcounts_l2()

Commit Message

Alberto Garcia June 28, 2020, 11:02 a.m. UTC

The offset field of an uncompressed cluster's L2 entry must be aligned
to the cluster size, otherwise it is invalid. If the cluster has no
data then it means that the offset points to a preallocation, so we
can clear the offset field without affecting the guest-visible data.
This is what 'qemu-img check' does when run in repair mode.

On traditional qcow2 images this can only happen when QCOW_OFLAG_ZERO
is set, and repairing such entries turns the clusters from ZERO_ALLOC
into ZERO_PLAIN.

Extended L2 entries have no ZERO_ALLOC clusters and no QCOW_OFLAG_ZERO
but the idea is the same: if none of the subclusters are allocated
then we can clear the offset field and leave the bitmap untouched.

Signed-off-by: Alberto Garcia <[e-mail]>
---
 block/qcow2-refcount.c     | 16 +++++++++++-----
 tests/qemu-iotests/060.out |  2 +-
 2 files changed, 12 insertions(+), 6 deletions(-)

Comments

Max Reitz July 2, 2020, 1:32 p.m. UTC | #1

On 28.06.20 13:02, Alberto Garcia wrote:
> The offset field of an uncompressed cluster's L2 entry must be aligned
> to the cluster size, otherwise it is invalid. If the cluster has no
> data then it means that the offset points to a preallocation, so we
> can clear the offset field without affecting the guest-visible data.
> This is what 'qemu-img check' does when run in repair mode.
> 
> On traditional qcow2 images this can only happen when QCOW_OFLAG_ZERO
> is set, and repairing such entries turns the clusters from ZERO_ALLOC
> into ZERO_PLAIN.
> 
> Extended L2 entries have no ZERO_ALLOC clusters and no QCOW_OFLAG_ZERO
> but the idea is the same: if none of the subclusters are allocated
> then we can clear the offset field and leave the bitmap untouched.
> 
> Signed-off-by: Alberto Garcia <[e-mail]>
> ---
>  block/qcow2-refcount.c     | 16 +++++++++++-----
>  tests/qemu-iotests/060.out |  2 +-
>  2 files changed, 12 insertions(+), 6 deletions(-)

Reviewed-by: Max Reitz <[e-mail]>

Patch

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 770c5dbc83..aae52607eb 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -1669,12 +1669,18 @@  static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
 
             /* Correct offsets are cluster aligned */
             if (offset_into_cluster(s, offset)) {
+                bool contains_data;
                 res->corruptions++;
 
-                if (qcow2_get_cluster_type(bs, l2_entry) ==
-                    QCOW2_CLUSTER_ZERO_ALLOC)
-                {
-                    fprintf(stderr, "%s offset=%" PRIx64 ": Preallocated zero "
+                if (has_subclusters(s)) {
+                    uint64_t l2_bitmap = get_l2_bitmap(s, l2_table, i);
+                    contains_data = (l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC);
+                } else {
+                    contains_data = !(l2_entry & QCOW_OFLAG_ZERO);
+                }
+
+                if (!contains_data) {
+                    fprintf(stderr, "%s offset=%" PRIx64 ": Preallocated "
                             "cluster is not properly aligned; L2 entry "
                             "corrupted.\n",
                             fix & BDRV_FIX_ERRORS ? "Repairing" : "ERROR",
@@ -1686,7 +1692,7 @@  static int check_refcounts_l2(BlockDriverState *bs, BdrvCheckResult *res,
                         int ign = active ? QCOW2_OL_ACTIVE_L2 :
                                            QCOW2_OL_INACTIVE_L2;
 
-                        l2_entry = QCOW_OFLAG_ZERO;
+                        l2_entry = has_subclusters(s) ? 0 : QCOW_OFLAG_ZERO;
                         set_l2_entry(s, l2_table, i, l2_entry);
                         ret = qcow2_pre_write_overlap_check(bs, ign,
                                 l2e_offset, l2_entry_size(s), false);
diff --git a/tests/qemu-iotests/060.out b/tests/qemu-iotests/060.out
index be5f8707a3..fa3d68f0df 100644
--- a/tests/qemu-iotests/060.out
+++ b/tests/qemu-iotests/060.out
@@ -320,7 +320,7 @@  discard 65536/65536 bytes at offset 0
 qcow2: Marking image as corrupt: Preallocated zero cluster offset 0x2a00 unaligned (guest offset: 0); further corruption events will be suppressed
 write failed: Input/output error
 --- Repairing ---
-Repairing offset=2a00: Preallocated zero cluster is not properly aligned; L2 entry corrupted.
+Repairing offset=2a00: Preallocated cluster is not properly aligned; L2 entry corrupted.
 The following inconsistencies were found and repaired:
 
     0 leaked clusters