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

[Jun Li]

On Mon, 09/01 13:11, Benoît Canet wrote:
> The Monday 01 Sep 2014 à 18:52:48 (+0800), Jun Li wrote :
> > When every item of refcount block is NULL, free refcount block and reset the
> > corresponding item of refcount table with NULL.
> > 
> > Signed-off-by: Jun Li <address@hidden>
> > ---
> > 
> > The v2 do following change to modify some potential issue.
> > 
> >              +------- Here should start from "0".
> >              |
> >     for (k = 0; k < refcount_block_entries; k++) {
> >         if (refcount_block[k] != cpu_to_be16(0)) {
> >         ...                |                 |
> >         }                  |                 |
> >     }                      |                 +---- Using "0" is more safe.
> >                            |
> >                            +-------- This should be "k" not "++k".
> > ---
> >  block/qcow2-refcount.c | 31 +++++++++++++++++++++++++++++++
> >  1 file changed, 31 insertions(+)
> > 
> > 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) {
> > +            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);
> > +                /* 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;
> > +                }
> > +
> 
> > +                s->refcount_table[refcount_table_index] = data64;
> 
> Shouldn't the in memory version be be in cpu order ? like
>         s->refcount_table[refcount_table_index] = 0;

I don't think so. See following:

(gdb) p sizeof(s->refcount_table[0])
$5 = 8
(gdb) p sizeof(s->refcount_table[1])
$6 = 8
(gdb) p sizeof(0)
$7 = 4

So I think here is right. Thank you for sharing Max's patch(qcow2: Drop
REFCOUNT_SHIFT) with me. I find this patch has been reviewed, but it has not
been merged. Maybe I will modify my realization after this patch merged.

Thanks again.

Jun Li


> 
> Best regards
> 
> Benoît 
> > +
> > +            }
> > +        }
> >      }
> >  
> >      ret = 0;
> > -- 
> > 1.9.3
> > 
> >