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

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Date:   Fri, 24 Aug 2018 11:25:29 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared
 pages

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