On 01/23/2015 11:12 AM, Vlastimil Babka wrote:
> On 01/23/2015 08:47 AM, 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.
>
> An other examples? What about zero pages?
I don't think this patch handles the zero page, due to
the reference count being higher than 1.
Handling the zero page could be a good next case to handle
in Ebru's OPW project to improve the THP collapse rate.
>> 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>
>
> Sounds like a good idea.
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> nits below:
>
>> ---
>> 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))
>
> Took me a while to grok this !!PageSwapCache(page) part. Perhaps expand
> the comment?
>
>> 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;
>
> Too much indent on the 2 lines above.
>
>> + }
>> + /*
>> + * Page is not in the swap cache, and page count is
>> + * one (see above). It can be collapsed into a THP.
>> + */
>
> Such comment sounds like a good place for:
>
> VM_BUG_ON(page_count(page) != 1));
>
>> + }
>> +
>> /*
>> * Isolate the page to avoid collapsing an hugepage
>> * currently in use by the VM.
>> @@ -2550,7 +2561,7 @@ 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 +2584,12 @@ 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;
>> @@ -2592,7 +2607,7 @@ static int khugepaged_scan_pmd(struct mm_struct
>> *mm,
>> 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)
>> + if (page_count(page) != 1 + !!PageSwapCache(page))
>
> Same as above. Even more so, as there's no other page swap cache
> handling code in this function.
>
> Thanks.
>
>> goto out_unmap;
>> if (pte_young(pteval) || PageReferenced(page) ||
>> mmu_notifier_test_young(vma->vm_mm, address))
>>
>
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