Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages

[Mike Kravetz]

On 08/27/2018 12:46 AM, Michal Hocko wrote:
> On Fri 24-08-18 11:08:24, Mike Kravetz wrote:
>> On 08/24/2018 01:41 AM, Michal Hocko wrote:
>>> On Thu 23-08-18 13:59:16, Mike Kravetz wrote:
>>>
>>> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>>>
>>> One nit below.
>>>
>>> [...]
>>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>>> index 3103099f64fd..a73c5728e961 100644
>>>> --- a/mm/hugetlb.c
>>>> +++ b/mm/hugetlb.c
>>>> @@ -4548,6 +4548,9 @@ static unsigned long page_table_shareable(struct vm_area_struct *svma,
>>>>  	return saddr;
>>>>  }
>>>>  
>>>> +#define _range_in_vma(vma, start, end) \
>>>> +	((vma)->vm_start <= (start) && (end) <= (vma)->vm_end)
>>>> +
>>>
>>> static inline please. Macros and potential side effects on given
>>> arguments are just not worth the risk. I also think this is something
>>> for more general use. We have that pattern at many places. So I would
>>> stick that to linux/mm.h
>>
>> Thanks Michal,
>>
>> Here is an updated patch which does as you suggest above.
> [...]
>> @@ -1409,6 +1419,32 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>  		subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
>>  		address = pvmw.address;
>>  
>> +		if (PageHuge(page)) {
>> +			if (huge_pmd_unshare(mm, &address, pvmw.pte)) {
>> +				/*
>> +				 * huge_pmd_unshare unmapped an entire PMD
>> +				 * page.  There is no way of knowing exactly
>> +				 * which PMDs may be cached for this mm, so
>> +				 * we must flush them all.  start/end were
>> +				 * already adjusted above to cover this range.
>> +				 */
>> +				flush_cache_range(vma, start, end);
>> +				flush_tlb_range(vma, start, end);
>> +				mmu_notifier_invalidate_range(mm, start, end);
>> +
>> +				/*
>> +				 * The ref count of the PMD page was dropped
>> +				 * which is part of the way map counting
>> +				 * is done for shared PMDs.  Return 'true'
>> +				 * here.  When there is no other sharing,
>> +				 * huge_pmd_unshare returns false and we will
>> +				 * unmap the actual page and drop map count
>> +				 * to zero.
>> +				 */
>> +				page_vma_mapped_walk_done(&pvmw);
>> +				break;
>> +			}
> 
> This still calls into notifier while holding the ptl lock. Either I am
> missing something or the invalidation is broken in this loop (not also
> for other invalidations).

As Jerome said ...

When creating this patch, I started by using the same flush/invalidation
routines used by the existing code.  This is because it is not obvious what
interfaces can be called in what context, and I didn't want to do anything
different.  The best 'documentation' are the comments in the mmu_notifier_ops
definition.

-- 
Mike Kravetz