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

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Date:   Wed, 22 Aug 2018 14:28:48 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        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 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?
-- 
Michal Hocko
SUSE Labs