linux-kernel - Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages

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Date:   Thu, 23 Aug 2018 00:05:07 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared
 pages

On Tue, Aug 21, 2018 at 06:10:42PM -0700, Mike Kravetz wrote:
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3103099f64fd..f085019a4724 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4555,6 +4555,9 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
>  
>  	/*
>  	 * check on proper vm_flags and page table alignment
> +	 *
> +	 * Note that this is the same check used in huge_pmd_sharing_possible.
> +	 * If you change one, consider changing both.

Should we have helper to isolate the check in one place?

>  	 */
>  	if (vma->vm_flags & VM_MAYSHARE &&
>  	    vma->vm_start <= base && end <= vma->vm_end)
> @@ -4562,6 +4565,43 @@ static bool vma_shareable(struct vm_area_struct *vma, unsigned long addr)
>  	return false;
>  }
>  
> +/*
> + * Determine if start,end range within vma could be mapped by shared pmd.
> + * If yes, adjust start and end to cover range associated with possible
> + * shared pmd mappings.
> + */
> +bool huge_pmd_sharing_possible(struct vm_area_struct *vma,
> +				unsigned long *start, unsigned long *end)
> +{
> +	unsigned long check_addr = *start;
> +	bool ret = false;
> +
> +	if (!(vma->vm_flags & VM_MAYSHARE))
> +		return ret;

Do we ever use return value? I don't see it.

And in this case function name is not really work...

> +	for (check_addr = *start; check_addr < *end; check_addr += PUD_SIZE) {
> +		unsigned long a_start = check_addr & PUD_MASK;
> +		unsigned long a_end = a_start + PUD_SIZE;
> +
> +		/*
> +		 * If sharing is possible, adjust start/end if necessary.
> +		 *
> +		 * Note that this is the same check used in vma_shareable.  If
> +		 * you change one, consider changing both.
> +		 */
> +		if (vma->vm_start <= a_start && a_end <= vma->vm_end) {
> +			if (a_start < *start)
> +				*start = a_start;
> +			if (a_end > *end)
> +				*end = a_end;
> +
> +			ret = true;
> +		}
> +	}
> +
> +	return ret;
> +}
> +

-- 
 Kirill A. Shutemov