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SubjectRe: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages
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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@suse.com>
>>>
>>> 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

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