Re: [PATCH v4] mm: oom: introduce cpuset oom

[Gang Li <gang.li@linux.dev>]

Apologize for the extremely delayed response. I was previously occupied
with work unrelated to the Linux kernel.

On 2023/4/11 22:36, Michal Hocko wrote:
> I believe it still wouldn't hurt to be more specific here.
> CONSTRAINT_CPUSET is rather obscure. Looking at this just makes my head
> spin.
>          /* Check this allocation failure is caused by cpuset's wall function */
>          for_each_zone_zonelist_nodemask(zone, z, oc->zonelist,
>                          highest_zoneidx, oc->nodemask)
>                  if (!cpuset_zone_allowed(zone, oc->gfp_mask))
>                          cpuset_limited = true;
> > Does this even work properly and why? prepare_alloc_pages sets
> oc->nodemask to current->mems_allowed but the above gives us
> cpuset_limited only if there is at least one zone/node that is not
> oc->nodemask compatible. So it seems like this wouldn't ever get set
> unless oc->nodemask got reset somewhere. This is a maze indeed.Is there

In __alloc_pages:
```
/*
  * Restore the original nodemask if it was potentially replaced with
  * &cpuset_current_mems_allowed to optimize the fast-path attempt.
  */
ac.nodemask = nodemask;
page = __alloc_pages_slowpath(alloc_gfp, order, &ac);

```

__alloc_pages set ac.nodemask back to mempolicy before call
__alloc_pages_slowpath.


> any reason why we cannot rely on __GFP_HARWALL here? Or should we

In prepare_alloc_pages:
```
if (cpusets_enabled()) {
	*alloc_gfp |= __GFP_HARDWALL;
	...
}
```

Since __GFP_HARDWALL is set as long as cpuset is enabled, I think we can
use it to determine if we are under the constraint of CPUSET.

But I have a question: Why we always set __GFP_HARDWALL when cpuset is
enabled, regardless of the value of cpuset.mem_hardwall?


Thanks,
Gang Li