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

[Michal Hocko <mhocko@kernel.org>]

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.

> 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