linux-kernel - Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise

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Date: Mon, 26 Feb 2024 09:41:33 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, 21cnbao@...il.com
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mhocko@...e.com, minchan@...nel.org, peterx@...hat.com,
 ryan.roberts@....com, shy828301@...il.com, songmuchun@...edance.com,
 wangkefeng.wang@...wei.com, zokeefe@...gle.com, fengwei.yin@...el.com
Subject: Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in
 madvise_free

On 26.02.24 09:37, Lance Yang wrote:
> Hey Barry,
> 
> Thanks for taking time to review!
> 
> On Mon, Feb 26, 2024 at 12:00 PM Barry Song <21cnbao@...il.com> wrote:
> [...]
>> On Mon, Feb 26, 2024 at 1:33 AM Lance Yang <ioworker0@...il.com> wrote:
> [...]
>> We did something similar on MADV_PAGEOUT[1]
>> [1] https://lore.kernel.org/linux-mm/20240118111036.72641-7-21cnbao@gmail.com/
> 
> Thanks for providing the link above.
> 
> [...]
>>> +                        * Avoid unnecessary folio splitting if the large
>>> +                        * folio is entirely within the given range.
>>> +                        */
>>> +                       folio_test_clear_dirty(folio);
>>> +                       folio_unlock(folio);
>>> +                       for (; addr != next_addr; pte++, addr += PAGE_SIZE) {
>>> +                               ptent = ptep_get(pte);
>>> +                               if (pte_young(ptent) || pte_dirty(ptent)) {
>>> +                                       ptent = ptep_get_and_clear_full(
>>> +                                               mm, addr, pte, tlb->fullmm);
>>> +                                       ptent = pte_mkold(ptent);
>>> +                                       ptent = pte_mkclean(ptent);
>>> +                                       set_pte_at(mm, addr, pte, ptent);
>>> +                                       tlb_remove_tlb_entry(tlb, pte, addr);
>>> +                               }
>>
>> The code works under the assumption the large folio is entirely mapped
>> in all PTEs in the range. This is not always true.
>>
>> This won't work in some cases as some PTEs might be mapping to the
>> large folios. some others might have been unmapped or mapped
>> to different folios.
>>
>> so in MADV_PAGEOUT, we have a function to check the folio is
>> really entirely mapped:
>>
>> +static inline bool pte_range_cont_mapped(unsigned long start_pfn,
>> + pte_t *start_pte, unsigned long start_addr, int nr)
>> +{
>> +              int i;
>> +              pte_t pte_val;
>> +
>> +              for (i = 0; i < nr; i++) {
>> +                           pte_val = ptep_get(start_pte + i);
>> +
>> +                           if (pte_none(pte_val))
>> +                                        return false;
>> +
>> +                           if (pte_pfn(pte_val) != (start_pfn + i))
>> +                                        return false;
>> +              }
>> +
>> +              return true;
>> +}
> 
> Thanks for providing the information; it's very helpful to me!
> I made some changes. Would you mind taking another look, please?
> 
> As a diff against this patch.
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index bcbf56595a2e..255d2f329be4 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -616,6 +616,18 @@ static long madvise_pageout(struct vm_area_struct *vma,
>   	return 0;
>   }
>   
> +static inline bool pte_range_cont_mapped(pte_t *pte, unsigned long nr)
> +{
> +	pte_t pte_val;
> +	unsigned long pfn = pte_pfn(pte);
> +	for (int i = 0; i < nr; i++) {
> +		pte_val = ptep_get(pte + i);
> +		if (pte_none(pte_val) || pte_pfn(pte_val) != (pfn + i))
> +			return false;
> +	}
> +	return true;
> +}

I dislike the "cont mapped" terminology.

Maybe folio_pte_batch() does what you want?

-- 
Cheers,

David / dhildenb