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

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Message-Id: <20170614161857.69d54338@mschwideX1>
Date:   Wed, 14 Jun 2017 16:18:57 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     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>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.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 Wed, 14 Jun 2017 16:51:43 +0300
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> 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>
> ...
> @@ -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);
> 
>  	if (freeze) {

That won't work on s390. After pmdp_invalidate the pmd entry is gone,
it has been replaced with _SEGMENT_ENTRY_EMPTY. This includes the
dirty and referenced bits. The old scheme is

        entry = *pmd;
        pmdp_invalidate(vma, addr, pmd);
	if (pmd_dirty(entry))
		...

Could we change pmdp_invalidate to make it return the old pmd entry?
The pmdp_xchg_direct function already returns it, for s390 that would
be an easy change. The above code snippet would change like this:

	entry = pmdp_invalidate(vma, addr, pmd);
	if (pmd_dirty(entry))
		...

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.