Commit acbfb087 authored Nov 30, 2019 by Zhigang Lu Committed by Linus Torvalds Dec 01, 2019

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>

parent 188b04a7

mm/hugetlb.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -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);

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