linux-kernel - Re: [PATCH 3/3] mm, thp: Do not loose dirty bit in __split_huge_pmd

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Date:   Wed, 14 Jun 2017 20:58:45 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm, thp: Do not loose dirty bit in
 __split_huge_pmd_locked()



On Wednesday 14 June 2017 07:21 PM, Kirill A. Shutemov wrote:
> Until pmdp_invalidate() pmd entry is present and CPU can update it,
> setting dirty. Currently, we tranfer dirty bit to page too early and
> there is window when we can miss dirty bit.
> 
> Let's call SetPageDirty() after pmdp_invalidate().
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>   mm/huge_memory.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index a84909cf20d3..c4ee5c890910 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1928,7 +1928,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>   	struct page *page;
>   	pgtable_t pgtable;
>   	pmd_t _pmd;
> -	bool young, write, dirty, soft_dirty;
> +	bool young, write, soft_dirty;
>   	unsigned long addr;
>   	int i;
> 
> @@ -1965,7 +1965,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>   	page_ref_add(page, HPAGE_PMD_NR - 1);
>   	write = pmd_write(*pmd);
>   	young = pmd_young(*pmd);
> -	dirty = pmd_dirty(*pmd);
>   	soft_dirty = pmd_soft_dirty(*pmd);
> 
>   	pmdp_huge_split_prepare(vma, haddr, pmd);
> @@ -1995,8 +1994,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>   			if (soft_dirty)
>   				entry = pte_mksoft_dirty(entry);
>   		}
> -		if (dirty)
> -			SetPageDirty(page + i);
>   		pte = pte_offset_map(&_pmd, addr);
>   		BUG_ON(!pte_none(*pte));
>   		set_pte_at(mm, addr, pte, entry);
> @@ -2046,6 +2043,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>   	 * pmd_populate.
>   	 */
>   	pmdp_invalidate(vma, haddr, pmd);
> +
> +	/*
> +	 * Transfer dirty bit to page after pmd invalidated, so CPU would not
> +	 * be able to set it under us.
> +	 */
> +	if (pmd_dirty(*pmd))
> +		SetPageDirty(page);
> +
>   	pmd_populate(mm, pmd, pgtable);
> 

you fixed dirty bit loosing i discussed in my previous mail here.

thanks
-aneesh