Re: [Qemu-devel] [PATCH] qcow2-refcount: Snapshot update for zero clusters

[Kevin Wolf <kwolf@redhat.com>]

Am 29.08.2013 um 14:56 hat Max Reitz geschrieben:
> Am 29.08.2013 14:53, schrieb Kevin Wolf:
> >Am 29.08.2013 um 14:38 hat Max Reitz geschrieben:
> >>Am 29.08.2013 14:33, schrieb Kevin Wolf:
> >>>Am 29.08.2013 um 12:16 hat Max Reitz geschrieben:
> >>>>Do not try to update the refcount for zero clusters in
> >>>>qcow2_update_snapshot_refcount.
> >>>>
> >>>>Signed-off-by: Max Reitz <mreitz@redhat.com>
> >>>>---
> >>>>  block/qcow2-refcount.c | 16 +++++++++++++---
> >>>>  1 file changed, 13 insertions(+), 3 deletions(-)
> >>>Please don't forget to add a test case for v2.
> >>Okay.
> >>
> >>>>diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> >>>>index 1244693..7555242 100644
> >>>>--- a/block/qcow2-refcount.c
> >>>>+++ b/block/qcow2-refcount.c
> >>>>@@ -863,9 +863,13 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> >>>>              for(j = 0; j < s->l2_size; j++) {
> >>>>                  offset = be64_to_cpu(l2_table[j]);
> >>>>                  if (offset != 0) {
> >>>>+                    uint64_t cluster_index;
> >>>>+
> >>>>                      old_offset = offset;
> >>>>                      offset &= ~QCOW_OFLAG_COPIED;
> >>>>-                    if (offset & QCOW_OFLAG_COMPRESSED) {
> >>>>+
> >>>>+                    switch (qcow2_get_cluster_type(offset)) {
> >>>>+                    case QCOW2_CLUSTER_COMPRESSED:
> >>>>                          nb_csectors = ((offset >> s->csize_shift) &
> >>>>                                         s->csize_mask) + 1;
> >>>>                          if (addend != 0) {
> >>>>@@ -880,8 +884,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> >>>>                          }
> >>>>                          /* compressed clusters are never modified */
> >>>>                          refcount = 2;
> >>>>-                    } else {
> >>>>-                        uint64_t cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits;
> >>>>+                        break;
> >>>>+
> >>>>+                    case QCOW2_CLUSTER_NORMAL:
> >>>>+                        cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits;
> >>>>                          if (addend != 0) {
> >>>>                              refcount = update_cluster_refcount(bs, cluster_index, addend,
> >>>>                                                                 QCOW2_DISCARD_SNAPSHOT);
> >>>>@@ -893,6 +899,10 @@ int qcow2_update_snapshot_refcount(BlockDriverState *bs,
> >>>>                              ret = refcount;
> >>>>                              goto fail;
> >>>>                          }
> >>>>+                        break;
> >>>I think this part isn't quite right. Even zero clusters can have an
> >>>allocated offset, so I think what's really needed is that the same code
> >>>runs from QCOW2_CLUSTER_ZERO. However, add an if (cluster_index == 0)
> >>>case for the unallocated case.
> >>So then it would be enough to change:
> >>
> >>- if (offset != 0) {
> >>+ if ((offset & L2E_OFFSET_MASK) != 0) {
> >>
> >>and just execute the same code for QCOW2_CLUSTER_NORMAL and
> >>QCOW2_CLUSTER_ZERO?
> >Doing only this change in the original code might happen to work, but
> >it's not really clean as the L2 entry format is different for compressed
> >clusters (L2E_OFFSET_MASK isn't valid for it). So having the switch for
> >the cluster type is a good thing, but the implementation for normal and
> >zero clusters should be shared indeed.
> Yes, of course I meant to include the switch as well.
> 
> >The offset != 0 check ends up redundant when you introduce the switch,
> >so in theory it could as well be removed.
> >
> Well, I don't know; for zero clusters, the if statement is required
> anyway (since qcow2_get_cluster_type does not distinguish between
> allocated and non-allocated zero clusters), so I though we might as
> well just leave it where it is (just slightly adjusted in the way I
> proposed).

You do need a zero check, but it's only for normal/zero clusters, not
for compressed ones; and it must check offset & L2E_OFFSET_MASK (or
cluster_index) rather than offset itself.

So I think you end up removing the outer if (offset != 0) and adding a
new inner check in case QCOW2_CLUSTER_NORMAL/ZERO.

Kevin