linux-kernel - Re: [PATCH v2] mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

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Date:   Tue, 3 Sep 2019 14:26:08 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Zhigang Lu <totty.lu@...il.com>, luzhigang001@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Zhigang Lu <tonnylu@...cent.com>
Subject: Re: [PATCH v2] mm/hugetlb: avoid looping to the same hugepage if
 !pages and !vmas

On 8/29/19 6:50 AM, Zhigang Lu wrote:
> From: Zhigang Lu <tonnylu@...cent.com>
> 
> 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@...cent.com>
> Reviewed-by: Haozhong Zhang <hzhongzhang@...cent.com>
> Reviewed-by: Zongming Zhang <knightzhang@...cent.com>
> Acked-by: Matthew Wilcox <willy@...radead.org>
> ---
>  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@...cle.com>
-- 
Mike Kravetz