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

[Gregory Price <gregory.price@memverge.com>]

On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@memverge.com> writes:
> 
> > On Wed, Jan 31, 2024 at 02:43:12PM +0800, Huang, Ying wrote:
> >> Gregory Price <gourry.memverge@gmail.com> 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.
>

That's probably worth just adding now, I'll do it and squash the style
updates into the branch.  Sorry Andrew, I guess 1 last version is
inbound :]

I'll pick up the reviewed tags along the way.

~Gregory