Re: [PATCH v4] mm/page_vma_mapped.c: Explicitly compare pfn for normal, hugetlbfs and THP page

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-01-14 at 17:25 Michal Hocko wrote:
>On Sat 11-01-20 10:18:05, Li Xinhai wrote:
>> When check_pte, pfn of normal, hugetlbfs and THP page need be compared.
>> The current implementation apply comparison as
>> - normal 4K page: page_pfn <= pfn < page_pfn + 1
>> - hugetlbfs page:  page_pfn <= pfn < page_pfn + HPAGE_PMD_NR
>> - THP page: page_pfn <= pfn < page_pfn + HPAGE_PMD_NR
>> in pfn_in_hpage. For hugetlbfs page, it should be
>> page_pfn == pfn
>>
>> Now, change pfn_in_hpage to pfn_is_match to highlight that comparison
>> is not only for THP and explicitly compare for these cases.
>
>Why is this important to do. I have asked and Mike had the same feeling
>that the patch is missing any real justification. Why do we need this
>change? It is great that you have dropped VM_BUG_ON btw.
> 
I think it is important to make the code clear, as said, comparing hugetlbfs page
in range page_pfn <= pfn < page_pfn + HPAGE_PMD_NR is confusion.

>> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
>> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> ---
>>  mm/page_vma_mapped.c | 12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
>> index eff4b45..719c352 100644
>> --- a/mm/page_vma_mapped.c
>> +++ b/mm/page_vma_mapped.c
>> @@ -52,12 +52,16 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
>>  return true;
>>  }
>> 
>> -static inline bool pfn_in_hpage(struct page *hpage, unsigned long pfn)
>> +static inline bool pfn_is_match(struct page *page, unsigned long pfn)
>>  {
>> -	unsigned long hpage_pfn = page_to_pfn(hpage);
>> +	unsigned long page_pfn = page_to_pfn(page);
>> +
>> +	/* normal page and hugetlbfs page */
>> +	if (!PageTransCompound(page) || PageHuge(page))
>> +	return page_pfn == pfn;
>> 
>>  /* THP can be referenced by any subpage */
>> -	return pfn >= hpage_pfn && pfn - hpage_pfn < hpage_nr_pages(hpage);
>> +	return pfn >= page_pfn && pfn - page_pfn < hpage_nr_pages(page);
>>  }
>> 
>>  /**
>> @@ -108,7 +112,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>>  pfn = pte_pfn(*pvmw->pte);
>>  }
>> 
>> -	return pfn_in_hpage(pvmw->page, pfn);
>> +	return pfn_is_match(pvmw->page, pfn);
>>  }
>> 
>>  /**
>> --
>> 1.8.3.1
>
>--
>Michal Hocko
>SUSE Labs