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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8624b7758abb665bf661bfe4f1d22036d3cb5844..ac65bb5e38ac267dec41c3e09bdaa53501da4c9a 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4338,6 +4338,21 @@ 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);