On Wed, Jan 31, 2024 at 05:19:51PM +0800, Huang, Ying wrote:
> 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.
>
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
