On Fri, 09/05 16:33, Stefan Hajnoczi wrote:
> On Mon, Sep 01, 2014 at 06:52:48PM +0800, Jun Li wrote:
>
> How does this patch handle self-describing refcount blocks? I think
> they will keep the refcount block alive forever because your code will
> not decide to free them.
>
Sorry, I have ignored self-describing refcount blocks. :)
> This patch should also discard the refcount block if we decide to free
> it (in the same way that we discard at cluster_offset).
>
> > diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> > index 43665b8..63f36e6 100644
> > --- a/block/qcow2-refcount.c
> > +++ b/block/qcow2-refcount.c
> > @@ -586,6 +586,37 @@ static int QEMU_WARN_UNUSED_RESULT
> > update_refcount(BlockDriverState *bs,
> > if (refcount == 0 && s->discard_passthrough[type]) {
> > update_refcount_discard(bs, cluster_offset, s->cluster_size);
> > }
> > +
> > + /* When refcount block is NULL, update refcount table */
> > + if (block_index == 0) {
>
> What is the purpose of block_index == 0?
Here is want to reduce the probability of running the following code. Only
when block_index == 0, we will run the following code to free refcount block.
>
> > + int k = block_index;
> > + int refcount_block_entries = s->cluster_size /
> > sizeof(uint16_t);
> > + for (k = 0; k < refcount_block_entries; k++) {
> > + if (refcount_block[k] != cpu_to_be16(0)) {
> > + break;
> > + }
> > + }
> > +
> > + if (k == refcount_block_entries) {
> > + qemu_vfree(refcount_block);
>
> You can't do this, the buffer belongs to the refcount block cache.
> Please look at the cache get/put as well as qcow2_cache_create/destroy.
ok, thx. Should add
qcow2_cache_put(bs, s->refcount_block_cache, (void**) refcount_block);
>
> > + /* update refcount table */
> > + unsigned int refcount_table_index;
> > + uint64_t data64 = cpu_to_be64(0);
> > + refcount_table_index = cluster_index >> (s->cluster_bits -
> > + REFCOUNT_SHIFT);
> > + ret = bdrv_pwrite_sync(bs->file,
> > + s->refcount_table_offset +
> > + refcount_table_index *
> > + sizeof(uint64_t),
> > + &data64, sizeof(data64));
> > + if (ret < 0) {
> > + goto fail;
> > + }
>
> Plase use write_reftable_entry().
ok, got it. I will submit a new version when I submit v3 of qcow2 shrinking.
Best Regards,
Jun Li
