Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

[Zi Yan <ziy@nvidia.com>]

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On 27 Mar 2020, at 13:05, Kirill A. Shutemov wrote:

> We can collapse PTE-mapped compound pages. We only need to avoid
> handling them more than once: lock/unlock page only once if it's present
> in the PMD range multiple times as it handled on compound level. The
> same goes for LRU isolation and putpack.

s/putpack/putback/

>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b47edfe57f7b..c8c2c463095c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
>
>  static void release_pte_page(struct page *page)
>  {
> +	/*
> +	 * We need to unlock and put compound page on LRU only once.
> +	 * The rest of the pages have to be locked and not on LRU here.
> +	 */
> +	VM_BUG_ON_PAGE(!PageCompound(page) &&
> +			(!PageLocked(page) && PageLRU(page)), page);
It only checks base pages.

Add

VM_BUG_ON_PAGE(PageCompound(page) && PageLocked(page) && PageLRU(page), page);

to check for compound pages.

> +
> +	if (!PageLocked(page))
> +		return;
> +
> +	page = compound_head(page);
>  	dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
>  	unlock_page(page);
>  	putback_lru_page(page);
> @@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	pte_t *_pte;
>  	int none_or_zero = 0, result = 0, referenced = 0;
>  	bool writable = false;
> +	LIST_HEAD(compound_pagelist);
>
>  	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
>  	     _pte++, address += PAGE_SIZE) {
> @@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  			goto out;
>  		}
>
> -		/* TODO: teach khugepaged to collapse THP mapped with pte */
> +		VM_BUG_ON_PAGE(!PageAnon(page), page);
> +
>  		if (PageCompound(page)) {
> -			result = SCAN_PAGE_COMPOUND;
> -			goto out;
> -		}
> +			struct page *p;
> +			page = compound_head(page);
>
> -		VM_BUG_ON_PAGE(!PageAnon(page), page);
> +			/*
> +			 * Check if we have dealt with the compount page

s/compount/compound/

> +			 * already
> +			 */
> +			list_for_each_entry(p, &compound_pagelist, lru) {
> +				if (page ==  p)
> +					break;
> +			}
> +			if (page ==  p)
> +				continue;
> +		}
>
>  		/*
>  		 * We can do it before isolate_lru_page because the
> @@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		    page_is_young(page) || PageReferenced(page) ||
>  		    mmu_notifier_test_young(vma->vm_mm, address))
>  			referenced++;
> +
> +		if (PageCompound(page))
> +			list_add_tail(&page->lru, &compound_pagelist);
>  	}
>  	if (likely(writable)) {
>  		if (likely(referenced)) {

Do we need a list here? There should be at most one compound page we will see here, right?

If a compound page is seen here, can we bail out the loop early? I guess not,
because we can a partially mapped compound page at the beginning or the end of a VMA, right?

> @@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  			goto out_unmap;
>  		}
>
> -		/* TODO: teach khugepaged to collapse THP mapped with pte */
> -		if (PageCompound(page)) {
> -			result = SCAN_PAGE_COMPOUND;
> -			goto out_unmap;
> -		}
> +		page = compound_head(page);
>
>  		/*
>  		 * Record which node the original page is from and save this
> -- 
> 2.26.0


—
Best Regards,
Yan Zi

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