linux-kernel - Re: [PATCH 1/2] mm/hugetlb: grab head page refcount once per group of subpages

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Date:   Mon, 25 Jan 2021 18:14:23 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Joao Martins <joao.m.martins@...cle.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] mm/hugetlb: grab head page refcount once per group of
 subpages

On 1/25/21 12:57 PM, Joao Martins wrote:
> follow_hugetlb_page() once it locks the pmd/pud, it checks all the
> subpages in a huge page and grabs a reference for each one,
> depending on how many pages we can store or the size of va range.
> Similar to gup-fast, have follow_hugetlb_page() grab the head
> page refcount only after counting all its subpages that are part
> of the just faulted huge page.
> 
> Consequently we reduce the number of atomics necessary to pin
> said huge page, which improves non-fast gup() considerably:
> 
>  - 16G with 1G huge page size
>  gup_test -f /mnt/huge/file -m 16384 -r 10 -L -S -n 512 -w
> 
>  PIN_LONGTERM_BENCHMARK: ~87.6k us -> ~11k us
> 
> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
> ---
>  include/linux/mm.h |  3 +++
>  mm/gup.c           |  5 ++---
>  mm/hugetlb.c       | 43 ++++++++++++++++++++++++-------------------
>  3 files changed, 29 insertions(+), 22 deletions(-)

Thanks.  Nice straight forward improvement.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz

> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a5d618d08506..0d793486822b 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1182,6 +1182,9 @@ static inline void get_page(struct page *page)
>  }
>  
>  bool __must_check try_grab_page(struct page *page, unsigned int flags);
> +__maybe_unused struct page *try_grab_compound_head(struct page *page, int refs,
> +						   unsigned int flags);
> +
>  
>  static inline __must_check bool try_get_page(struct page *page)
>  {
> diff --git a/mm/gup.c b/mm/gup.c
> index 3e086b073624..ecadc80934b2 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -79,9 +79,8 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
>   * considered failure, and furthermore, a likely bug in the caller, so a warning
>   * is also emitted.
>   */
> -static __maybe_unused struct page *try_grab_compound_head(struct page *page,
> -							  int refs,
> -							  unsigned int flags)
> +__maybe_unused struct page *try_grab_compound_head(struct page *page,
> +						   int refs, unsigned int flags)
>  {
>  	if (flags & FOLL_GET)
>  		return try_get_compound_head(page, refs);
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a6bad1f686c5..016addc8e413 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4798,7 +4798,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  	unsigned long vaddr = *position;
>  	unsigned long remainder = *nr_pages;
>  	struct hstate *h = hstate_vma(vma);
> -	int err = -EFAULT;
> +	int err = -EFAULT, refs;
>  
>  	while (vaddr < vma->vm_end && remainder) {
>  		pte_t *pte;
> @@ -4918,26 +4918,11 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			continue;
>  		}
>  
> +		refs = 0;
> +
>  same_page:
> -		if (pages) {
> +		if (pages)
>  			pages[i] = mem_map_offset(page, pfn_offset);
> -			/*
> -			 * try_grab_page() should always succeed here, because:
> -			 * a) we hold the ptl lock, and b) we've just checked
> -			 * that the huge page is present in the page tables. If
> -			 * the huge page is present, then the tail pages must
> -			 * also be present. The ptl prevents the head page and
> -			 * tail pages from being rearranged in any way. So this
> -			 * page must be available at this point, unless the page
> -			 * refcount overflowed:
> -			 */
> -			if (WARN_ON_ONCE(!try_grab_page(pages[i], flags))) {
> -				spin_unlock(ptl);
> -				remainder = 0;
> -				err = -ENOMEM;
> -				break;
> -			}
> -		}
>  
>  		if (vmas)
>  			vmas[i] = vma;
> @@ -4946,6 +4931,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		++pfn_offset;
>  		--remainder;
>  		++i;
> +		refs++;
>  		if (vaddr < vma->vm_end && remainder &&
>  				pfn_offset < pages_per_huge_page(h)) {
>  			/*
> @@ -4953,6 +4939,25 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  			 * of this compound page.
>  			 */
>  			goto same_page;
> +		} else if (pages) {
> +			/*
> +			 * try_grab_compound_head() should always succeed here,
> +			 * because: a) we hold the ptl lock, and b) we've just
> +			 * checked that the huge page is present in the page
> +			 * tables. If the huge page is present, then the tail
> +			 * pages must also be present. The ptl prevents the
> +			 * head page and tail pages from being rearranged in
> +			 * any way. So this page must be available at this
> +			 * point, unless the page refcount overflowed:
> +			 */
> +			if (WARN_ON_ONCE(!try_grab_compound_head(pages[i-1],
> +								 refs,
> +								 flags))) {
> +				spin_unlock(ptl);
> +				remainder = 0;
> +				err = -ENOMEM;
> +				break;
> +			}
>  		}
>  		spin_unlock(ptl);
>  	}
>