Re: [PATCH] mm: mempolicy: fix policy_nodemask() for MPOL_PREFERRED_MANY case

[Feng Tang]

On Tue, Aug 02, 2022 at 05:02:37PM +0800, Michal Hocko wrote:
> Please make sure to CC Mike on hugetlb related changes.

OK.

> I didn't really get to grasp your proposed solution but it feels goind
> sideways. The real issue is that hugetlb uses a dedicated allocation
> scheme which is not fully MPOL_PREFERRED_MANY aware AFAICS. I do not
> think we should be tricking that by providing some fake nodemasks and
> what not.
> 
> The good news is that allocation from the pool is MPOL_PREFERRED_MANY
> aware because it first tries to allocation from the preffered node mask
> and then fall back to the full nodemask (dequeue_huge_page_vma).
> If the existing pools cannot really satisfy that allocation then it
> tries to allocate a new hugetlb page (alloc_fresh_huge_page) which also
> performs 2 stage allocation with the node mask and no node masks. But
> both of them might fail.
> 
> The bad news is that other allocation functions - including those that
> allocate to the pool are not fully MPOL_PREFERRED_MANY aware. E.g.
> __nr_hugepages_store_common paths which use the allocating process
> policy to fill up the pool so the pool could be under provisioned if
> that context is using MPOL_PREFERRED_MANY.

Thanks for the check!

So you mean if the prferred nodes don't have enough pages, we should
also fallback to all like dequeue_huge_page_vma() does?

Or we can user a policy API which return nodemask for MPOL_BIND and 
NULL for all other policies, like allowed_mems_nr() needs.

--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -158,6 +158,18 @@ static inline nodemask_t *policy_nodemask_current(gfp_t gfp)
 	return policy_nodemask(gfp, mpol);
 }
 
+#ifdef CONFIG_HUGETLB_FS
+static inline nodemask_t *strict_policy_nodemask_current(void)
+{
+	struct mempolicy *mpol = get_task_policy(current);
+
+	if (mpol->mode == MPOL_BIND)
+		return &mpol->nodes;
+
+	return NULL;
+}
+#endif
+

> Wrt. allowed_mems_nr (i.e. hugetlb_acct_memory) this is a reservation
> code and I have to admit I do not really remember details there. This is
> a subtle code and my best guess would be that policy_nodemask_current
> should be hugetlb specific and only care about MPOL_BIND.

The API needed by allowed_mem_nr() is a little different as it has gfp
flag and cpuset config to consider.

Thanks,
Feng

[snip]