[patch 132/158] mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

[akpm@linux-foundation.org]

From: Zhigang Lu <tonnylu@tencent.com>
Subject: mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

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.

Link: http://lkml.kernel.org/r/1567581712-5992-1-git-send-email-totty.lu@gmail.com
Signed-off-by: Zhigang Lu <tonnylu@tencent.com>
Reviewed-by: Haozhong Zhang <hzhongzhang@tencent.com>
Reviewed-by: Zongming Zhang <knightzhang@tencent.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/hugetlb.c~mm-hugetlb-avoid-looping-to-the-same-hugepage-if-pages-and-vmas
+++ a/mm/hugetlb.c
@@ -4338,6 +4338,21 @@ long follow_hugetlb_page(struct mm_struc
 				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);
_