Re: [PATCH v2] mm: incorporate read-only pages into transparent huge pages

[Vlastimil Babka <vbabka@suse.cz>]

On 01/24/2015 04:38 PM, Ebru Akagunduz wrote:
> This patch aims to improve THP collapse rates, by allowing
> THP collapse in the presence of read-only ptes, like those
> left in place by do_swap_page after a read fault.
> 
> Currently THP can collapse 4kB pages into a THP when
> there are up to khugepaged_max_ptes_none pte_none ptes
> in a 2MB range. This patch applies the same limit for
> read-only ptes.
> 
> The patch was tested with a test program that allocates
> 800MB of memory, writes to it, and then sleeps. I force
> the system to swap out all but 190MB of the program by
> touching other memory. Afterwards, the test program does
> a mix of reads and writes to its memory, and the memory
> gets swapped back in.
> 
> Without the patch, only the memory that did not get
> swapped out remained in THPs, which corresponds to 24% of
> the memory of the program. The percentage did not increase
> over time.
> 
> With this patch, after 5 minutes of waiting khugepaged had
> collapsed 48% of the program's memory back into THPs.
> 
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> Reviewed-by: Rik van Riel <riel@redhat.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> Changes in v2:
>  - Remove extra code indent
>  - Add fast path optimistic check to
>    __collapse_huge_page_isolate()

My interpretation is that the optimistic check is in khugepaged_scan_pmd() while
in __collapse_huge_page_isolate() it's protected by lock, as Andrea suggested?

>  - Add comment line for check condition of page_count()
>  - Move check condition of page_count() below to trylock_page()
> 
> I've written down test results:
> With the patch:
> After swapped out:
> cat /proc/pid/smaps:
> Anonymous:	42804 kB
> AnonHugePages:	38912 kB
> Swap:		757200 kB
> Fraction:	90,90
> 
> cat /proc/meminfo:
> AnonPages:	1843956 kB
> AnonHugePages:	1712128 kB
> Fraction:	92,85
> 
> After swapped in:
> In a few seconds:
> cat /proc/pid/smaps:
> Anonymous:	800004 kB
> AnonHugePages:	104448 kB
> Swap:		0 kB
> Fraction:	13,05
> 
> cat /proc/meminfo:
> AnonPages:	2605728 kB
> AnonHugePages:	1777664 kB
> Fraction:	68,22
> 
> In 5 minutes:
> cat /proc/pid/smaps
> Anonymous:	800004 kB
> AnonHugePages:	389120 kB
> Swap:		0 kB
> Fraction:	48,63
> 
> cat /proc/meminfo:
> AnonPages:	2607824 kB
> AnonHugePages:	2041856 kB
> Fraction:	78,29
> 
> Without the patch:
> After swapped out:
> cat /proc/pid/smaps:
> Anonymous:      190660 kB
> AnonHugePages:  190464 kB
> Swap:           609344 kB
> Fraction:       99,89
> 
> cat /proc/meminfo:
> AnonPages:      1740456 kB
> AnonHugePages:  1667072 kB
> Fraction:       95,78
> 
> After swapped in:
> cat /proc/pid/smaps:
> Anonymous:      800004 kB
> AnonHugePages:  190464 kB
> Swap:           0 kB
> Fraction:       23,80
> 
> cat /proc/meminfo:
> AnonPages:      2350032 kB
> AnonHugePages:  1667072 kB
> Fraction:       70,93
> 
> I waited 10 minutes the fractions
> did not change without the patch.
> 
>  mm/huge_memory.c | 48 +++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 39 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 817a875..5e3e9b9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2148,7 +2148,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  {
>  	struct page *page;
>  	pte_t *_pte;
> -	int referenced = 0, none = 0;
> +	int referenced = 0, none = 0, ro = 0;
>  	for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
>  	     _pte++, address += PAGE_SIZE) {
>  		pte_t pteval = *_pte;
> @@ -2158,7 +2158,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  			else
>  				goto out;
>  		}
> -		if (!pte_present(pteval) || !pte_write(pteval))
> +		if (!pte_present(pteval))
>  			goto out;
>  		page = vm_normal_page(vma, address, pteval);
>  		if (unlikely(!page))
> @@ -2168,9 +2168,6 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		VM_BUG_ON_PAGE(!PageAnon(page), page);
>  		VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
>  
> -		/* cannot use mapcount: can't collapse if there's a gup pin */
> -		if (page_count(page) != 1)
> -			goto out;
>  		/*
>  		 * We can do it before isolate_lru_page because the
>  		 * page can't be freed from under us. NOTE: PG_lock
> @@ -2179,6 +2176,31 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  		 */
>  		if (!trylock_page(page))
>  			goto out;
> +
> +		/*
> +		 * cannot use mapcount: can't collapse if there's a gup pin.
> +		 * The page must only be referenced by the scanned process
> +		 * and page swap cache.
> +		 */
> +		if (page_count(page) != 1 + !!PageSwapCache(page)) {
> +			unlock_page(page);
> +			goto out;
> +		}
> +		if (!pte_write(pteval)) {
> +			if (++ro > khugepaged_max_ptes_none) {
> +				unlock_page(page);
> +				goto out;

So just for completeness, as I said later for v1 I think this can leave us with
read-only VMA: consider ro == 256 and none == 256, referenced can still be >0
(up to 256). I think that the check for referenced that follows this for loop
should also check if (ro + none < HPAGE_PMD_NR).

> +			}
> +			if (PageSwapCache(page) && !reuse_swap_page(page)) {
> +				unlock_page(page);
> +				goto out;
> +			}
> +			/*
> +			 * Page is not in the swap cache, and page count is
> +			 * one (see above). It can be collapsed into a THP.
> +			 */

I would still put the VM_BUG_ON(page_count(page) != 1) here as I suggested
previously. Even more so that I think it would have been able to catch the
problem that Andrea pointed out in v1.

> +		}
> +
>  		/*
>  		 * Isolate the page to avoid collapsing an hugepage
>  		 * currently in use by the VM.
> @@ -2550,7 +2572,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  {
>  	pmd_t *pmd;
>  	pte_t *pte, *_pte;
> -	int ret = 0, referenced = 0, none = 0;
> +	int ret = 0, referenced = 0, none = 0, ro = 0;
>  	struct page *page;
>  	unsigned long _address;
>  	spinlock_t *ptl;
> @@ -2573,8 +2595,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  			else
>  				goto out_unmap;
>  		}
> -		if (!pte_present(pteval) || !pte_write(pteval))
> +		if (!pte_present(pteval))
>  			goto out_unmap;
> +		if (!pte_write(pteval)) {
> +			if (++ro > khugepaged_max_ptes_none)
> +				goto out_unmap;
> +		}
>  		page = vm_normal_page(vma, _address, pteval);
>  		if (unlikely(!page))
>  			goto out_unmap;
> @@ -2591,8 +2617,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  		VM_BUG_ON_PAGE(PageCompound(page), page);
>  		if (!PageLRU(page) || PageLocked(page) || !PageAnon(page))
>  			goto out_unmap;
> -		/* cannot use mapcount: can't collapse if there's a gup pin */
> -		if (page_count(page) != 1)
> +		/*
> +		 * cannot use mapcount: can't collapse if there's a gup pin.
> +		 * The page must only be referenced by the scanned process
> +		 * and page swap cache.
> +		 */
> +		if (page_count(page) != 1 + !!PageSwapCache(page))
>  			goto out_unmap;
>  		if (pte_young(pteval) || PageReferenced(page) ||
>  		    mmu_notifier_test_young(vma->vm_mm, address))
> 

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