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.- 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.
I don't see conceptually better solution than what is proposed.