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Date: Mon, 26 Jan 2015 08:36:06 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org
CC: akpm@...ux-foundation.org, kirill@...temov.name, mhocko@...e.cz,
mgorman@...e.de, rientjes@...gle.com, sasha.levin@...cle.com,
hughd@...gle.com, hannes@...xchg.org, linux-kernel@...r.kernel.org,
riel@...hat.com, aarcange@...hat.com
Subject: Re: [PATCH v2] mm: incorporate read-only pages into transparent huge
pages
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@...il.com>
> Reviewed-by: Rik van Riel <riel@...hat.com>
> Acked-by: Vlastimil Babka <vbabka@...e.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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