mm, thp: allow fallback when pte_alloc_one() fails for huge pmd

The transparent hugepages feature is careful to not invoke the oom
killer when a hugepage cannot be allocated.

pte_alloc_one() failing in __do_huge_pmd_anonymous_page(), however,
currently results in VM_FAULT_OOM which invokes the pagefault oom killer
to kill a memory-hogging task.

This is unnecessary since it's possible to drop the reference to the
hugepage and fallback to allocating a small page.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -640,11 +640,8 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
 
 	VM_BUG_ON(!PageCompound(page));
 	pgtable = pte_alloc_one(mm, haddr);
-	if (unlikely(!pgtable)) {
-		mem_cgroup_uncharge_page(page);
-		put_page(page);
+	if (unlikely(!pgtable))
 		return VM_FAULT_OOM;
-	}
 
 	clear_huge_page(page, haddr, HPAGE_PMD_NR);
 	__SetPageUptodate(page);
@@ -723,8 +720,14 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			put_page(page);
 			goto out;
 		}
+		if (unlikely(__do_huge_pmd_anonymous_page(mm, vma, haddr, pmd,
+							  page))) {
+			mem_cgroup_uncharge_page(page);
+			put_page(page);
+			goto out;
+		}
 
-		return __do_huge_pmd_anonymous_page(mm, vma, haddr, pmd, page);
+		return 0;
 	}
 out:
 	/*