Re: [Qemu-devel] [PATCH v2] qcow2: add update refcount table realization for update_refcount

[Jun Li]

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