Re: [PATCH] mm/oom_kill: kill current in OOM when binding to cpu-less nodes

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Thu, 4 Sep 2025 16:36:28 +0200 Michal Hocko <mhocko@suse.com> wrote:

> On Thu 04-09-25 07:26:25, Joshua Hahn wrote:
> > On Thu, 4 Sep 2025 21:44:31 +0800 Jinjiang Tu <tujinjiang@huawei.com> wrote:
> > 
> > Hello Jinjiang,
> > 
> > I hope you are doing well, thank you for this patchset!
> > 
> > > out_of_memory() selects tasks without considering mempolicy. Assuming a
> > > cpu-less NUMA Node, ordinary process that don't set mempolicy don't
> > > allocate memory from this cpu-less Node, unless other NUMA Nodes are below
> > > low watermark. If a task binds to this cpu-less Node and triggers OOM, many
> > > tasks may be killed wrongly that don't occupy memory from this Node.
> > 
> > I am wondeirng whether you have seen this happen in practice, or if this is
> > just based on inspecting the code. I have a feeling that the case you are
> > concerned about may already be covered in select_bad_process.
> > 
> > out_of_memory(oc)
> >     select_bad_process(oc)
> >         oom_evaluate_task(p, oc)
> > 	    oom_cpuset_eligible(task, oc)
> > 	    
> > 	        [...snip...]
> > 
> > 		for_each_thread(start, tsk) {
> > 		    if (mask) {
> > 		        ret = mempolicy_in_oom_domain(tsk, mask);
> > 		    } else {
> > 		        ret = cpuset_mems_allowed_intersects(current, tsk)
> > 		    }
> > 		}
> > 
> > While iterating through the list of candidate processes, we check whether
> > oc->nodemask exists, and if not, we check if the nodemasks intersects. It seems
> > like these are the two checks that you add in the helper function.
> > 
> > With that said, I might be missing something obvious -- please feel to
> > correct me if I am misunderstanding your patch or if I'm missing something
> > in the existing oom target selection : -)
> 
> The thing with mempolicy_in_oom_domain is that it doesn't really do what
> you might be thinking it is doing ;) as it will true also for tasks
> without any NUMA affinity because those intersect with the given mask by
> definition as they can allocate from any node. So they are eligible and
> that is what Jinjiang Tu is considered about I believe.

Hello Michal! Thank you for your insights : -)

Looking back, I made the mistake of thinking that we cared about the
!oc->nodemask case, where Jinjiang's patch cares about the oc->nodemask == True
case. So I was checking that cpuset_mems_allowed_intersects was the same as
nodes_intersects, whereas I should have been checking if mempolicy_in_oom_domain
is correct.

Looking into it, everything you said is correct and I think I defintely
overlooked what the patch was trying to do. Thank you for clarifying these
points for me!

I hope you have a great day,
Joshua

> -- 
> Michal Hocko
> SUSE Labs