linux-kernel - Re: [PATCHv2 2/3] mm: Do not loose dirty and access bits in pmdp

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Date:   Fri, 16 Jun 2017 15:40:41 +0200
From:   Andrea Arcangeli <aarcange@...hat.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>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/3] mm: Do not loose dirty and access bits in
 pmdp_invalidate()

On Thu, Jun 15, 2017 at 05:52:23PM +0300, Kirill A. Shutemov wrote:
> -void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
> +pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  		     pmd_t *pmdp)
>  {
> -	pmd_t entry = *pmdp;
> -	set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));
> -	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +	pmd_t old = pmdp_establish(pmdp, pmd_mknotpresent(*pmdp));
> +	if (pmd_present(old))
> +		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +	return old;
>  }
>  #endif

The pmd_present() check added above is superflous because there's no
point to call pmdp_invalidate if the pmd is not present (present as in
pmd_present) already. pmd_present returns true if _PAGE_PSE is set
and it was always set before calling pmdp_invalidate.

It looks like we could skip the flush if _PAGE_PRESENT is not set
(i.e. for example if the pmd is PROTNONE) but that's not what the above
pmd_present will do.