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

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?

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?