Re: [Qemu-block] [PATCH 1/6] qcow2: use one single memory block for the L2/refcount cache tables

[Kevin Wolf]

Am 05.05.2015 um 12:28 hat Stefan Hajnoczi geschrieben:
> On Mon, May 04, 2015 at 12:58:13PM +0200, Kevin Wolf wrote:
> > Am 01.05.2015 um 16:23 hat Stefan Hajnoczi geschrieben:
> > > On Thu, Apr 30, 2015 at 01:11:40PM +0300, Alberto Garcia wrote:
> > > >  Qcow2Cache *qcow2_cache_create(BlockDriverState *bs, int num_tables)
> > > >  {
> > > >      BDRVQcowState *s = bs->opaque;
> > > >      Qcow2Cache *c;
> > > > -    int i;
> > > >  
> > > >      c = g_new0(Qcow2Cache, 1);
> > > >      c->size = num_tables;
> > > > +    c->table_size = s->cluster_size;
> > > 
> > > This assumes c->table_size meets bs' memory alignment requirements.  The
> > > following would be safer:
> > > 
> > > c->table_size = QEMU_ALIGN_UP(c->table_size, 
> > > bdrv_opt_mem_align(bs->file));
> > 
> > You can't just access more than one cluster. You might be caching data
> > and later write it back when it's stale.
> 
> I don't mean I/O alignment, just memory alignment (i.e. the start
> address of the data buffer in memory).

The start address is already taken care of by qemu_blockalign(). With
rounding c->table_size, you'd align the length, and that would be wrong.

Though looking at the code again I see now that c->table_size isn't
consistently used. The I/O requests still use s->cluster_size. We should
either use it everywhere or not introduce it at all.

Kevin

pgpzwATVF7Jjx.pgp
Description: PGP signature