Re: [PATCH 2/2] mm/mempolicy: stop copying the nodemask in the interleave paths

[Rakie Kim]

On Fri, 4 Sep 2026 11:50:47 -0400 Gregory Price <gourry@xxxxxxxxxx> wrote:

> On Fri, Sep 04, 2026 at 05:01:37PM +0900, Rakie Kim wrote:
> >
> > What I had in mind is the gap between the two reads. Say the policy
> > starts with two nodes, every weight is 10, and 100 pages are
> > requested:
> >
> > 	/* the mask is {0,1} here */
> > 	do {
> > 		cpuset_mems_cookie = read_mems_allowed_begin();
> > 		nnodes = nodes_weight(pol->nodes);	/* nnodes = 2 */
> > 	} while (read_mems_allowed_retry(cpuset_mems_cookie));
> >
> > 	/* a rebind grows the mask to {0,1,2,3} at this point */
> >
> > 	/* calculate total, detect system default usage */
> > 	for_each_node_mask(node, pol->nodes)
> > 		weight_total += ...;			/* 10 * 4 = 40 */
> >
> > 	rounds = rem_pages / weight_total;		/* 100 / 40 = 2 */
> >
> > 	for (i = 0; i < nnodes; i++)			/* bounded by 2 */
> > 		...
> >
>
> Consider:
>
> 	/* nodemask: {0,1} */
>  	for_each_node_mask(node, pol->nodes) {
>  		weight_total += ...;
> 		nnodes++;
> 	}
>
>  	/* a rebind grows the mask to {2,3,4,5} */
>
>  	rounds = rem_pages / weight_total;		/* 100 / 10 = 10 */
>  	for (i = 0; i < nnodes; i++)			/* bounded by 2 */
>  		...
>
> in this scenario every value is wrong.  The weight total was calculated
> based on {0,1} and the loop will use {2,3} weights and ignore {4,5}
> entirely.
>
> It's the nature of the mechanism and race - best we can do is ensure
> safety.  Ensuring correct distributions would likely require locks or
> reworking the entire weight mechanism.
>
> I'd rather keep the change simple (cookie the value that can cause a
> div/0) and leave the math alone.
>
> ~Gregory

Fair enough. Your example makes the point. The mask can change after
the sum as well, so matching the two reads does not get us a correct
distribution either. Keeping it simple sounds right to me.

Please feel free to add the following to this patch:
Reviewed-by: Rakie Kim <rakie.kim@xxxxxx>

Thanks for walking through it.

Rakie Kim