Re: [PATCH] mempolicy: fix div-by-zero in alloc_pages_bulk_weighted_interleave

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 02, 2026 at 02:40:39PM -0700, Andrew Morton wrote:
> On Wed, 2 Sep 2026 09:57:35 -0400 Gregory Price <gourry@gourry.net> wrote:
> 
> > On Wed, Sep 02, 2026 at 05:04:12PM +0800, Liu Jing wrote:
> > > In alloc_pages_bulk_weighted_interleave(), if the iw_table contains
> > > all-zero weights for every node in the policy nodemask, weight_total
> > > sums to zero. The subsequent "rem_pages / weight_total" and
> > > "rem_pages % weight_total" trigger a divide-by-zero panic.
> > > 
> > 
> > weights can't be zero and we just fixed the rebind race
> 
> So if the caller passes in me->weight==0 then we consider that caller
> to be buggy.
> 

basically weights cannot be 0 by construction:

What:           /sys/kernel/mm/mempolicy/weighted_interleave/nodeN
...
                The minimum weight for a node is always 1.

What:           /sys/kernel/mm/mempolicy/weighted_interleave/auto
...
                If they were not previously set or are onlined with missing
		bandwidth data, the weights will use a default weight of 1.


int mempolicy_set_node_perf(unsigned int node, struct access_coordinate *coords)
{       
...
        new_wi_state->mode_auto = true;
        for (i = 0; i < nr_node_ids; i++)
                new_wi_state->iw_table[i] = 1;


static ssize_t node_store(struct kobject *kobj, struct kobj_attribute *attr,
                          const char *buf, size_t count)
{
...
                for (i = 0; i < nr_node_ids; i++)
                        new_wi_state->iw_table[i] = 1;


static ssize_t weighted_interleave_auto_store(struct kobject *kobj,
                struct kobj_attribute *attr, const char *buf, size_t count)
{
...
        for (i = 0; i < nr_node_ids; i++)
                new_wi_state->iw_table[i] = 1;


we default all weights to 1 and never let calculation drop below 1 for
exactly this div/0 reason.

The div/0 risk came solely from the torn nodemask read issue.

~Gregory