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

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Date:   Wed, 22 Aug 2018 14:48:31 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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 08/22/2018 02:05 PM, Kirill A. Shutemov wrote:
> 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?
> 

Yes, I will create one.  Most likely just a #define.

>>  	 */
>>  	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...

You are correct.  None of the code uses the return value.  I initially
thought some caller would use it.  But every caller wants/needs to
adjust the range if sharing is possible.  This is a really long name
but how about:

void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
				unsigned long *start, unsigned long *end)

I'm open to other names and will update patch with suggestions.
-- 
Mike Kravetz

> 
>> +	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;
>> +}
>> +