Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages

[Michal Hocko]

On Thu 23-08-18 13:59:16, Mike Kravetz wrote:
[...]
> @@ -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;
> +			}
> +		}

Wait a second. This is not correct, right? You have to call the
notifiers after page_vma_mapped_walk_done because they might be
sleepable and we are still holding the pte lock. This is btw. a problem
for other users of mmu_notifier_invalidate_range in try_to_unmap_one,
unless I am terribly confused. This would suggest 369ea8242c0fb is
incorrect.
-- 
Michal Hocko
SUSE Labs