Re: [Qemu-devel] [PATCH v3.1 25/31] hostmem: add properties for NUMA memory policy

[Eduardo Habkost]

On Fri, Jun 06, 2014 at 11:37:26AM +0800, Hu Tao wrote:
> On Mon, May 19, 2014 at 08:34:54PM -0300, Eduardo Habkost wrote:
> > On Tue, May 06, 2014 at 05:27:46PM +0800, Hu Tao wrote:
> > [...]
> > > @@ -203,6 +296,20 @@ host_memory_backend_memory_init(UserCreatable *uc, 
> > > Error **errp)
> > >      if (backend->prealloc) {
> > >          os_mem_prealloc(memory_region_get_fd(&backend->mr), ptr, sz);
> > >      }
> > > +
> > > +#ifdef CONFIG_NUMA
> > > +    unsigned long maxnode = find_last_bit(backend->host_nodes, 
> > > MAX_NODES);
> > > +
> > > +    /* This is a workaround for a long standing bug in Linux'
> > > +     * mbind implementation, which cuts off the last specified
> > > +     * node.
> > > +     */
> > 
> > What if the bug is fixed? mbind() documentation says "nodemask points to
> 
> No it won't, otherwise softwares depend on mbind() will break.
> 
> > a bit mask of nodes containing up to maxnode bits", so we must ensure
> > backend->host_nodes has the one extra bit.
> 
> Yes.
> 
> > 
> > Also, if no bit is set, we can pass nodemask=NULL or maxnode=0 as
> > argument.
> > 
> > We could address both issues, and do this:
> > 
> >     struct HostMemoryBackend { [...]
> >         DECLARE_BITMAP(host_nodes, MAX_NODES + 1);
> >     [...]
> >     lastbit = find_last_bit(backend->host_nodes, MAX_NODES);
> >     /* lastbit == MAX_NODES means maxnode=0 */
> >     maxnode = (lastbit + 1) % (MAX_NODES + 1);
> >     /* We can have up to MAX_NODES nodes, but we need to pass maxnode+1
> >      * as argument to mbind() due to an old Linux bug (feature?) which
> >      * cuts off the last specified node. This means backend->host_nodes
> >      * must have MAX_NODES+1 bits available.
> >      */
> >     assert(sizeof(backend->host_nodes) >= BITS_TO_LONGS(MAX_NODES + 1) * 
> > sizeof(unsigned long));
> >     assert(maxnode <= MAX_NODES);
> 
> I think we can just omit these two asserts since they are guaranteed to
> be true.

asserts() must be always guaranteed to be true, that's the whole point
of using them. They can detect subtle off-by-one bugs if somebody
introduces them in the future.

> 
> >     mbind(ptr, sz, policy, maxnode ? backend->host_nodes : NULL, maxnode + 
> > 1, flags);
> > 
> > 
> > (I am starting to wonder if it was worth dropping the libnuma
> > requirement and implementing our own mbind()-calling code.)
> > 
> > > +    if (mbind(ptr, sz, backend->policy, backend->host_nodes, maxnode + 
> > > 2, 0)) {
> > > +        error_setg_errno(errp, errno,
> > > +                         "cannot bind memory to host NUMA nodes");
> > 
> > Don't we want to set flags to MPOL_MF_STRICT here? I believe we
> > shouldn't have any pages preallocated at this point, but in case we do,
> > I would expect them to be moved instead of ignoring the policy set by
> > the user.
> 
> MPOL_MF_STRICT | MPOL_MF_MOVE to move. Actually in this version the
> preallocation happens before mbind, which is fixed in v3.2.

If memory was already allocated in a different node and has to be moved
that early, that's a bug we want to detect and fix (instead of
triggering useles memory moves). So I would use only MPOL_MF_STRICT.

-- 
Eduardo