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

I do not know how often PMD sharing is exercised. It certainly is used by
DBs for large shared areas. I suspect it is less frequent than hugtlb pages
in general, and certainly less frequent than THP or base pages.

>>
>> while (page_vma_mapped_walk(&pvmw)) {
>> @@ -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 is documented to require a pte lock. Where do we take
> it?

It is somewhat hidden, but we are in the loop:

while (page_vma_mapped_walk(&pvmw)) {

The routine page_vma_mapped_walk will acquire the lock, and it correctly
checks for huge pages and uses huge_pte_lockptr().

page_vma_mapped_walk_done() will release the lock.
--
Mike Kravetz

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