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
