On 08/23/2018 01:21 AM, Kirill A. Shutemov wrote:
> On Thu, Aug 23, 2018 at 09:30:35AM +0200, Michal Hocko wrote:
>> On Wed 22-08-18 09:48:16, Mike Kravetz wrote:
>>> On 08/22/2018 05:28 AM, Michal Hocko wrote:
>>>> On Tue 21-08-18 18:10:42, Mike Kravetz wrote:
>>>> [...]
>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>> index eb477809a5c0..8cf853a4b093 100644
>>>>> --- a/mm/rmap.c
>>>>> +++ b/mm/rmap.c
>>>>> @@ -1362,11 +1362,21 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
>>>>> }
>>>>>
>>>>> /*
>>>>> - * We have to assume the worse case ie pmd for invalidation. Note that
>>>>> - * the page can not be free in this function as call of try_to_unmap()
>>>>> - * must hold a reference on the page.
>>>>> + * For THP, we have to assume the worse case ie pmd for invalidation.
>>>>> + * For hugetlb, it could be much worse if we need to do pud
>>>>> + * invalidation in the case of pmd sharing.
>>>>> + *
>>>>> + * Note that the page can not be free in this function as call of
>>>>> + * try_to_unmap() must hold a reference on the page.
>>>>> */
>>>>> end = min(vma->vm_end, start + (PAGE_SIZE << compound_order(page)));
>>>>> + if (PageHuge(page)) {
>>>>> + /*
>>>>> + * If sharing is possible, start and end will be adjusted
>>>>> + * accordingly.
>>>>> + */
>>>>> + (void)huge_pmd_sharing_possible(vma, &start, &end);
>>>>> + }
>>>>> mmu_notifier_invalidate_range_start(vma->vm_mm, start, end);
>>>>
>>>> I do not get this part. Why don't we simply unconditionally invalidate
>>>> the whole huge page range?
>>>
>>> In this routine, we are only unmapping a single page. The existing code
>>> is limiting the invalidate range to that page size: 4K or 2M. With shared
>>> PMDs, we have the possibility of unmapping a PUD_SIZE area: 1G. I don't
>>> think we want to unconditionally invalidate 1G. Is that what you are asking?
>>
>> But we know that huge_pmd_unshare unmapped a shared pte so we know when
>> to flush 2MB or 1GB. I really do not like how huge_pmd_sharing_possible
>> a) duplicates some checks and b) it updates start/stop out of line.
>
> My reading on this is that mmu_notifier_invalidate_range_start() has to be
> called from sleepable context on the full range that *can* be invalidated
> before following mmu_notifier_invalidate_range_end().
>
> In this case huge_pmd_unshare() may unmap aligned PUD_SIZE around the PMD
> page that effectively enlarge range that has to be covered by
> mmu_notifier_invalidate_range_start(). We cannot yet know if there's any
> shared page tables in the range, so we need to go with worst case
> scenario.
Yes, that is a good summary. We can not know for sure if there is PMD
sharing until we hold the page table lock. So, we don't know if we should
invalidate/flush 2M or 1G. Yet, we need to call
mmu_notifier_invalidate_range_start before taking the lock. And, the notifiers
need to know the range of the worst possible case. The best approach I came up
with is to adjust the range if sharing is 'possible'.
>
> I don't see conceptually better solution than what is proposed.
>
I have updated the patches based on Kirill's previous comments and will send out
a new version later today.
--
Mike Kravetz
