Re: [RESEND Patch v2 3/4] mm/mremap: calculate extent in one place

[Wei Yang <richard.weiyang@linux.alibaba.com>]

On Mon, Jul 06, 2020 at 01:07:29PM +0300, Kirill A. Shutemov wrote:
>On Fri, Jun 26, 2020 at 09:52:15PM +0800, Wei Yang wrote:
>> Page tables is moved on the base of PMD. This requires both source
>> and destination range should meet the requirement.
>> 
>> Current code works well since move_huge_pmd() and move_normal_pmd()
>> would check old_addr and new_addr again. And then return to move_ptes()
>> if the either of them is not aligned.
>> 
>> In stead of calculating the extent separately, it is better to calculate
>> in one place, so we know it is not necessary to try move pmd. By doing
>> so, the logic seems a little clear.
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
>> Tested-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>  mm/mremap.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index de27b12c8a5a..a30b3e86cc99 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -258,6 +258,9 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>  		extent = next - old_addr;
>>  		if (extent > old_end - old_addr)
>>  			extent = old_end - old_addr;
>> +		next = (new_addr + PMD_SIZE) & PMD_MASK;
>
>Please use round_up() for both 'next' calculations.
>

I took another close look into this, seems this is not a good suggestion.

   round_up(new_addr, PMD_SIZE)

would be new_addr when new_addr is PMD_SIZE aligned, which is not what we
expect.

>> +		if (extent > next - new_addr)
>> +			extent = next - new_addr;
>>  		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
>>  		if (!old_pmd)
>>  			continue;
>> @@ -301,9 +304,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>>  
>>  		if (pte_alloc(new_vma->vm_mm, new_pmd))
>>  			break;
>> -		next = (new_addr + PMD_SIZE) & PMD_MASK;
>> -		if (extent > next - new_addr)
>> -			extent = next - new_addr;
>>  		move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
>>  			  new_pmd, new_addr, need_rmap_locks);
>>  	}
>> -- 
>> 2.20.1 (Apple Git-117)
>> 
>
>-- 
> Kirill A. Shutemov

-- 
Wei Yang
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