Re: [Qemu-devel] [PATCH v6 09/11] qcow2: Clean up after refcount rebuild

[Kevin Wolf]

Am 21.10.2014 um 16:55 hat Max Reitz geschrieben:
> On 2014-10-21 at 12:16, Max Reitz wrote:
> >On 2014-10-21 at 11:59, Kevin Wolf wrote:
> >>Am 20.10.2014 um 16:35 hat Max Reitz geschrieben:
> >>>Because the old refcount structure will be leaked after having rebuilt
> >>>it, we need to recalculate the refcounts and run a leak-fixing
> >>>operation
> >>>afterwards (if leaks should be fixed at all).
> >>>
> >>>Signed-off-by: Max Reitz <address@hidden>
> >>>Reviewed-by: Benoît Canet <address@hidden>
> >>>---
> >>>  block/qcow2-refcount.c | 35 +++++++++++++++++++++++++++++++++++
> >>>  1 file changed, 35 insertions(+)
> >>>
> >>>diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> >>>index 75e726b..3730be2 100644
> >>>--- a/block/qcow2-refcount.c
> >>>+++ b/block/qcow2-refcount.c
> >>>@@ -1956,12 +1956,47 @@ int
> >>>qcow2_check_refcounts(BlockDriverState *bs, BdrvCheckResult
> >>>*res,
> >>>                        nb_clusters);
> >>>        if (rebuild && (fix & BDRV_FIX_ERRORS)) {
> >>>+        BdrvCheckResult old_res = *res;
> >>>+
> >>>          fprintf(stderr, "Rebuilding refcount structure\n");
> >>>          ret = rebuild_refcount_structure(bs, res, &refcount_table,
> >>>                                           &nb_clusters);
> >>>          if (ret < 0) {
> >>>              goto fail;
> >>>          }
> >>>+
> >>>+        res->corruptions = 0;
> >>>+        res->leaks = 0;
> >>>+
> >>>+        /* Because the old reftable has been exchanged for a
> >>>new one the
> >>>+         * references have to be recalculated */
> >>>+        rebuild = false;
> >>>+        memset(refcount_table, 0, nb_clusters * sizeof(uint16_t));
> >>>+        ret = calculate_refcounts(bs, res, 0, &rebuild,
> >>>&refcount_table,
> >>>+                                  &nb_clusters);
> >>>+        if (ret < 0) {
> >>>+            goto fail;
> >>>+        }
> >>>+
> >>>+        if (fix & BDRV_FIX_LEAKS) {
> >>>+            /* The old refcount structures are now leaked,
> >>>fix it; the result
> >>>+             * can be ignored */
> >>>+            pre_compare_res = *res;
> >>I would prefer using another local variable here. At the first sight
> >>it's not quite clear which references to pre_compare_res correspond to
> >>which state.
> >
> >Why not.
> >
> >>>+            compare_refcounts(bs, res, BDRV_FIX_LEAKS, &rebuild,
> >>>+                              &highest_cluster,
> >>>refcount_table, nb_clusters);
> >>>+            if (rebuild) {
> >>>+                fprintf(stderr, "ERROR rebuilt refcount
> >>>structure is still "
> >>>+                        "broken\n");
> >>>+            }
> >>>+            *res = pre_compare_res;
> >>>+        }
> >>>+
> >>>+        if (res->corruptions < old_res.corruptions) {
> >>>+            res->corruptions_fixed += old_res.corruptions -
> >>>res->corruptions;
> >>>+        }
> >>>+        if (res->leaks < old_res.leaks) {
> >>>+            res->leaks_fixed += old_res.leaks - res->leaks;
> >>>+        }
> >>For these numbers to be accurate, don't we need to run
> >>compare_refcounts() unconditionally and only make BDRV_FIX_LEAKS
> >>conditional?
> >
> >Actually, there is no difference, because at the point of this
> >patch, you cannot use BDRV_FIX_ERRORS without BDRV_FIX_LEAKS. But
> >it'd be more correct, right.
> 
> Wait, it would not be more correct. The result of the
> compare_refcounts() call inside of the "if (fix & BDRV_FIX_LEAKS)"
> conditional block is ignored, its only purpose is to fix leaks
> resulting from rebuild_refcount_structure().
> 
> So the question is whether we should discard the result of that
> compare_refcounts() call. I think we should. Its sole purpose is to
> fix leaks due to the rebuilt refcount structures, and qemu-img will
> double check anyway.

Right, the other leaks should have been fixed by rebuilding the refcount
structures. So what you're saying is that we could do this:

    if (res->corruptions < old_res.corruptions) {
        res->corruptions_fixed += old_res.corruptions - res->corruptions;
    }
    assert(res->leaks == 0);
    res->leaks_fixed = old_res.leaks;

If this weren't true, we'd ignore leaked clusters even with
BDRV_FIX_LEAKS set.

Kevin