On Fri, Jan 23, 2015 at 09:47:36AM +0200, 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 55% of the program's memory back into THPs.
>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
> I've written down test results:
>
> With the patch:
> After swapped out:
> cat /proc/pid/smaps:
> Anonymous: 100352 kB
> AnonHugePages: 98304 kB
> Swap: 699652 kB
> Fraction: 97,95
>
> cat /proc/meminfo:
> AnonPages: 1763732 kB
> AnonHugePages: 1716224 kB
> Fraction: 97,30
>
> After swapped in:
> In a few seconds:
> cat /proc/pid/smaps
> Anonymous: 800004 kB
> AnonHugePages: 235520 kB
> Swap: 0 kB
> Fraction: 29,43
>
> cat /proc/meminfo:
> AnonPages: 2464336 kB
> AnonHugePages: 1853440 kB
> Fraction: 75,21
>
> In five minutes:
> cat /proc/pid/smaps:
> Anonymous: 800004 kB
> AnonHugePages: 440320 kB
> Swap: 0 kB
> Fraction: 55,0
>
> cat /proc/meminfo:
> AnonPages: 2464340
> AnonHugePages: 2058240
> Fraction: 83,52
>
> 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 | 25 ++++++++++++++++++++-----
> 1 file changed, 20 insertions(+), 5 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 817a875..af750d9 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -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))
> @@ -2169,7 +2169,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
>
> /* cannot use mapcount: can't collapse if there's a gup pin */
> - if (page_count(page) != 1)
> + if (page_count(page) != 1 + !!PageSwapCache(page))
> goto out;
> /*
> * We can do it before isolate_lru_page because the
> @@ -2179,6 +2179,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> */
> if (!trylock_page(page))
> goto out;
> + if (!pte_write(pteval)) {
> + 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.
> + */
> + }
Hm. As a side effect it will effectevely allow collapse in PROT_READ vmas,
right? I'm not convinced it's a good idea.
--
Kirill A. Shutemov
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