On 8/29/19 6:50 AM, Zhigang Lu wrote:
> From: Zhigang Lu <tonnylu@xxxxxxxxxxx>
>
> 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.
Thanks for the analysis!
Just curious, do you have an application that does this (mmap(MAP_POPULATE)
for an existing hugetlbfs file), or was this part of some test suite or
debug code?
> Signed-off-by: Zhigang Lu <tonnylu@xxxxxxxxxxx>
> Reviewed-by: Haozhong Zhang <hzhongzhang@xxxxxxxxxxx>
> Reviewed-by: Zongming Zhang <knightzhang@xxxxxxxxxxx>
> Acked-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> ---
> mm/hugetlb.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6d7296d..2df941a 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4391,6 +4391,17 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> break;
> }
> }
It might be helpful to add a comment here to help readers of the code.
Something like:
/*
* 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);
>
With a comment added to the code,
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
--
Mike Kravetz
