On Thu, Aug 29, 2013 at 11:43:42AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 29, 2013 at 10:28:29AM +0100, Mel Gorman wrote:
> > On Fri, Aug 23, 2013 at 08:15:46PM +0200, Peter Zijlstra wrote:
> > > On Fri, Aug 23, 2013 at 03:03:32PM +0200, Peter Zijlstra wrote:
> > > > So I think this patch is broken (still).
> >
> > I am assuming the lack of complaints is that it is not a heavily executed
> > path. I expect that you (and Rik) are hitting this as part of automatic
> > NUMA balancing. Still a bug, just slightly less urgent if NUMA balancing
> > is the reproduction case.
>
> I thought it was, we crashed somewhere suspiciously close, but no. You
> need shared mpols for this to actually trigger and the NUMA stuff
> doesn't use that.
>
Ah, so this is a red herring?
> > > + if (unlikely((unsigned)nid >= MAX_NUMNODES))
> > > + goto again;
> > > +
> >
> > MAX_NUMNODES is unrelated to anything except that it might prevent a crash
> > and even then nr_online_nodes is probably what you wanted and even that
> > assumes the NUMA node numbering is contiguous.
>
> I used whatever nodemask.h did to detect end-of-bitmap and they use
> MAX_NUMNODES. See __next_node() and for_each_node() like.
>
The check does prevent us going off the end of the bitmap but does not
necessarily return an online node.
> MAX_NUMNODES doesn't assume contiguous numbers since its the actual size
> of the bitmap, nr_online_nodes would hoever.
>
I intended to say nr_node_ids, the same size as buffers such as the
task_numa_buffers. If we ever return a nid > nr_node_ids here then
task_numa_fault would corrupt memory. However, it should be possible for
node_weight to exceed nr_node_ids except maybe during node hot-remove so
it's not the problem.
> > The real concern is whether
> > the updated mask is an allowed target for the updated memory policy. If
> > it's not then "nid" can be pointing off the deep end somewhere. With this
> > conversion to a for loop there is race after you check nnodes where target
> > gets set to 0 and then return a nid of -1 which I suppose will just blow
> > up differently but it's fixable.
>
> But but but, I did i <= target, which will match when target == 0 so
> you'll get at least a single iteration and nid will be set.
>
True.
> > This? Untested. Fixes implicit types while it's there. Note the use of
> > first node and (c < target) to guarantee nid gets set and that the first
> > potential node is still used as an interleave target.
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 7431001..ae880c3 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -1755,22 +1755,24 @@ unsigned slab_node(void)
> > }
> >
> > /* Do static interleaving for a VMA with known offset. */
> > -static unsigned offset_il_node(struct mempolicy *pol,
> > +static unsigned int offset_il_node(struct mempolicy *pol,
> > struct vm_area_struct *vma, unsigned long off)
> > {
> > - unsigned nnodes = nodes_weight(pol->v.nodes);
> > - unsigned target;
> > - int c;
> > - int nid = -1;
> > + unsigned int nr_nodes, target;
> > + int i, nid;
> >
> > - if (!nnodes)
> > +again:
> > + nr_nodes = nodes_weight(pol->v.nodes);
> > + if (!nr_nodes)
> > return numa_node_id();
> > - target = (unsigned int)off % nnodes;
> > - c = 0;
> > - do {
> > + target = (unsigned int)off % nr_nodes;
> > + for (i = 0, nid = first_node(pol->v.nodes); i < target; i++)
> > nid = next_node(nid, pol->v.nodes);
> > - c++;
> > - } while (c <= target);
> > +
> > + /* Policy nodemask can potentially update in parallel */
> > + if (unlikely(!node_isset(nid, pol->v.nodes)))
> > + goto again;
> > +
> > return nid;
> > }
>
> So I explicitly didn't use the node_isset() test because that's more
> likely to trigger than the nid >= MAX_NUMNODES test. Its fine to return
> a node that isn't actually part of the mask anymore -- a race is a race
> anyway.
Yeah and as long as it's < nr_node_ids it should be ok within the task
numa fault handling as well.
--
Mel Gorman
SUSE Labs
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