On Wed, Jan 24, 2024 at 09:51:20AM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@xxxxxxxxxxxx> writes:
>
> + if (new && (new->mode == MPOL_INTERLEAVE ||
> + new->mode == MPOL_WEIGHTED_INTERLEAVE))
> current->il_prev = MAX_NUMNODES-1;
> task_unlock(current);
> mpol_put(old);
>
> I don't think we need to change this.
>
Ah you're right it's set to MAX_NUMNODES-1 here, but NUMA_NO_NODE can be
passed in as an argument to alloc_pages_bulk_array_mempolicy, like here:
vm_area_alloc_pages()
if (IS_ENABLED(CONFIG_NUMA) && nid == NUMA_NO_NODE)
nr = alloc_pages_bulk_array_mempolicy(bulk_gfp,
nr_pages_request,
pages + nr_allocated);
> > (cur_weight = 0) can happen in two scenarios:
> > - initial setting of mempolicy (NUMA_NO_NODE w/ cur_weight=0)
> > - weighted_interleave_nodes decrements it down to 0
> >
> > Now that i'm looking at it - the second condition should not exist, and
> > we can eliminate it. The logic in weighted_interleave_nodes is actually
> > annoyingly unclear at the moment, so I'm going to re-factor it a bit to
> > be more explicit.
>
> I am OK with either way. Just a reminder, the first condition may be
> true in alloc_pages_bulk_array_weighted_interleave() and perhaps some
> other places.
>
Yeah, the bulk allocator handles it correctly, it's just a matter of
clarity for weighted_interleave_nodes.
What isn't necessarily handled correctly is the rebind code. Rebind due
to a cgroup/mems_allowed change can cause a stale weight to be carried.
Basically cur_weight is not cleared, but the node it applied to may no
longer be the next node when next_node_in() is called.
The race condition is 1) exceedingly rare, and 2) not necessarily harmful,
just inaccurate. The worst case scenario is that a node receives up to 255
additional allocations once after a rebind (but more likely 10-20).
I was considering forcing the interleave forward like this:
@@ -356,6 +361,10 @@ static void mpol_rebind_nodemask(struct mempolicy *pol, const nodemask_t *nodes)
tmp = *nodes;
pol->nodes = tmp;
+
+ /* Weighted interleave policies are forced forward to the next node */
+ if (pol->mode & MPOL_WEIGHTED_INTERLEAVE)
+ pol->wil.cur_weight = 0;
}
But this creates 2 race conditions when we read cur_weight and nodemask
in the allocator path.
Example 1:
1) bulk allocator READ_ONCE(mask), READ_ONCE(cur_weight)
2) rebind changes nodemask and { cur_weight = 0; }
3) bulk allocator sets pol->wil.cur_weight
In this scenario, resume_weight is stale coming out of bulk allocations
if the resume_node has been removed from the node mask.
Example 2:
1) rebind changes nodemask
2) bulk allocator READ_ONCE(mask), READ_ONCE(cur_weight)
3) rebind sets { cur_weight = 0; }
In this scenario, cur_weight is stale going into bulk allocations.
Neither of these can force a violation of mems_allowed, just a
mis-application of a weight.
I'll need to think on this a bit. We can either leave this as-is,
meaning the first allocation after a rebind may apply the wrong weight
to a node, or we can try to track the current-interleave-node and
validate next_node_in(mask) == current-interleave-node before leaving
the allocator path (this may also be just as racey).
turns out concurrent counting is still hard :]
~Gregory
