linux-kernel - Re: [PATCHv5 05/28] mm: adjust FOLL

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Date:	Fri, 15 May 2015 13:05:27 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
CC:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv5 05/28] mm: adjust FOLL_SPLIT for new refcounting

On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> We need to prepare kernel to allow transhuge pages to be mapped with
> ptes too. We need to handle FOLL_SPLIT in follow_page_pte().
>
> Also we use split_huge_page() directly instead of split_huge_page_pmd().
> split_huge_page_pmd() will gone.

You still call split_huge_page_pmd() for the is_huge_zero_page(page) 
case. Also, of the code around split_huge_page() you basically took from 
split_huge_page_pmd() and open-coded into follow_page_mask(), you didn't 
include the mmu notifier calls. Why are they needed in 
split_huge_page_pmd() but not here?

>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Tested-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>   mm/gup.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++-----------------
>   1 file changed, 49 insertions(+), 18 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 203781fa96a5..ebdb39b3e820 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -79,6 +79,19 @@ retry:
>   		page = pte_page(pte);
>   	}
>
> +	if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> +		int ret;
> +		get_page(page);
> +		pte_unmap_unlock(ptep, ptl);
> +		lock_page(page);
> +		ret = split_huge_page(page);
> +		unlock_page(page);
> +		put_page(page);
> +		if (ret)
> +			return ERR_PTR(ret);
> +		goto retry;
> +	}
> +
>   	if (flags & FOLL_GET)
>   		get_page_foll(page);
>   	if (flags & FOLL_TOUCH) {
> @@ -186,27 +199,45 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
>   	}
>   	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
>   		return no_page_table(vma, flags);
> -	if (pmd_trans_huge(*pmd)) {
> -		if (flags & FOLL_SPLIT) {
> +	if (likely(!pmd_trans_huge(*pmd)))
> +		return follow_page_pte(vma, address, pmd, flags);
> +
> +	ptl = pmd_lock(mm, pmd);
> +	if (unlikely(!pmd_trans_huge(*pmd))) {
> +		spin_unlock(ptl);
> +		return follow_page_pte(vma, address, pmd, flags);
> +	}
> +
> +	if (unlikely(pmd_trans_splitting(*pmd))) {
> +		spin_unlock(ptl);
> +		wait_split_huge_page(vma->anon_vma, pmd);
> +		return follow_page_pte(vma, address, pmd, flags);
> +	}
> +
> +	if (flags & FOLL_SPLIT) {
> +		int ret;
> +		page = pmd_page(*pmd);
> +		if (is_huge_zero_page(page)) {
> +			spin_unlock(ptl);
> +			ret = 0;
>   			split_huge_page_pmd(vma, address, pmd);
> -			return follow_page_pte(vma, address, pmd, flags);
> -		}
> -		ptl = pmd_lock(mm, pmd);
> -		if (likely(pmd_trans_huge(*pmd))) {
> -			if (unlikely(pmd_trans_splitting(*pmd))) {
> -				spin_unlock(ptl);
> -				wait_split_huge_page(vma->anon_vma, pmd);
> -			} else {
> -				page = follow_trans_huge_pmd(vma, address,
> -							     pmd, flags);
> -				spin_unlock(ptl);
> -				*page_mask = HPAGE_PMD_NR - 1;
> -				return page;
> -			}
> -		} else
> +		} else {
> +			get_page(page);
>   			spin_unlock(ptl);
> +			lock_page(page);
> +			ret = split_huge_page(page);
> +			unlock_page(page);
> +			put_page(page);
> +		}
> +
> +		return ret ? ERR_PTR(ret) :
> +			follow_page_pte(vma, address, pmd, flags);
>   	}
> -	return follow_page_pte(vma, address, pmd, flags);
> +
> +	page = follow_trans_huge_pmd(vma, address, pmd, flags);
> +	spin_unlock(ptl);
> +	*page_mask = HPAGE_PMD_NR - 1;
> +	return page;
>   }
>
>   static int get_gate_page(struct mm_struct *mm, unsigned long address,
>

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