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

[Mike Kravetz <mike.kravetz@oracle.com>]

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;
>> +}
>> +