Re: [PATCH v4 3/3] mm/mempolicy: introduce MPOL_WEIGHTED_INTERLEAVE for weighted interleaving

[Huang, Ying]

Gregory Price <gregory.price@xxxxxxxxxxxx> writes:

> On Wed, Jan 31, 2024 at 02:43:12PM +0800, Huang, Ying wrote:
>> Gregory Price <gourry.memverge@xxxxxxxxx> writes:
>> >  
>> > +static unsigned int weighted_interleave_nodes(struct mempolicy *policy)
>> > +{
>> > +	unsigned int node = current->il_prev;
>> > +
>> > +	if (!current->il_weight || !node_isset(node, policy->nodes)) {
>> > +		node = next_node_in(node, policy->nodes);
>> > +		/* can only happen if nodemask is being rebound */
>> > +		if (node == MAX_NUMNODES)
>> > +			return node;
>> 
>> I feel a little unsafe to read policy->nodes at same time of writing in
>> rebound.  Is it better to use a seqlock to guarantee its consistency?
>> It's unnecessary to be a part of this series though.
>> 
>
> I think this is handled already? It is definitely an explicit race
> condition that is documented elsewhere:
>
> /*
>  * mpol_rebind_policy - Migrate a policy to a different set of nodes
>  *
>  * Per-vma policies are protected by mmap_lock. Allocations using per-task
>  * policies are protected by task->mems_allowed_seq to prevent a premature
>  * OOM/allocation failure due to parallel nodemask modification.
>  */

Thanks for pointing this out!

If we use task->mems_allowed_seq reader side in
weighted_interleave_nodes() we can guarantee the consistency of
policy->nodes.  That may be not deserved, because it's not a big deal to
allocate 1 page in a wrong node.

It makes more sense to do that in
alloc_pages_bulk_array_weighted_interleave(), because a lot of pages may
be allocated there.

> example from slub:
>
> do {
> 	cpuset_mems_cookie = read_mems_allowed_begin();
> 	zonelist = node_zonelist(mempolicy_slab_node(), pc->flags);
> 	...
> } while (read_mems_allowed_retry(cpuset_mems_cookie));
>
> quick perusal through other allocators, show similar checks.
>
> page_alloc.c  -  check_retry_cpusetset()
> filemap.c     -  filemap_alloc_folio()
>
> If we ever want mempolicy to be swappable from outside the current task
> context, this will have to change most likely - but that's another
> feature for another day.
>

--
Best Regards,
Huang, Ying