On Fri, Feb 07, 2020 at 09:18:07PM +0000, Chris Down wrote:

quoted

@@ -6856,8 +6857,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,

		}
	}

-	if (!memcg)
-		memcg = get_mem_cgroup_from_mm(mm);
+	if (!memcg) {
+		if (!mm)
+			memcg = get_mem_cgroup_from_current();
+		else
+			memcg = get_mem_cgroup_from_mm(mm);
+	}

Just to do due diligence, did we double check whether this results in any
unintentional shift in accounting for those passing in both mm and memcg as
NULL with no current->active_memcg set, since previously we never even tried
to consult current->mm and always used root_mem_cgroup in
get_mem_cgroup_from_mm?

Excellent question on a subtle issue.

But nobody actually passes NULL. They either pass current->mm (or a
destination mm) in syscalls, or vma->vm_mm in page faults.

The only times we end up with NULL is when kernel threads do something
and have !current->mm. We redirect those to root_mem_cgroup.

So this patch doesn't change those semantics.