mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

From: Zhigang Lu <[email protected]>

When mmapping an existing hugetlbfs file with MAP_POPULATE, we find it is very time consuming. For example, mmapping a 128GB file takes about 50 milliseconds. Sampling with perfevent shows it spends 99% time in the same_page loop in follow_hugetlb_page().

samples: 205 of event 'cycles', Event count (approx.): 136686374
- 99.04% test_mmap_huget [kernel.kallsyms] [k] follow_hugetlb_page

        follow_hugetlb_page
        __get_user_pages
        __mlock_vma_pages_range
        __mm_populate
        vm_mmap_pgoff
        sys_mmap_pgoff
        sys_mmap
        system_call_fastpath
        __mmap64

follow_hugetlb_page() is called with pages=NULL and vmas=NULL, so for each hugepage, we run into the same_page loop for pages_per_huge_page() times, but doing nothing. With this change, it takes less then 1 millisecond to mmap a 128GB file in hugetlbfs.

---
 mm/hugetlb.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

index 6d7296d..a096fb3 100644
@@ -4391,6 +4391,21 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				break;
 			}
 		}
+
+		/*
+		 * If subpage information not requested, update counters
+		 * and skip the same_page loop below.
+		 */
+		if (!pages && !vmas && !pfn_offset &&
+		    (vaddr + huge_page_size(h) < vma->vm_end) &&
+		    (remainder >= pages_per_huge_page(h))) {
+			vaddr += huge_page_size(h);
+			remainder -= pages_per_huge_page(h);
+			i += pages_per_huge_page(h);
+			spin_unlock(ptl);
+			continue;
+		}
+
 same_page:
 		if (pages) {
 			pages[i] = mem_map_offset(page, pfn_offset);

1.8.3.1