linux-kernel - Re: [PATCHv2 1/3] x86/mm: Provide pmdp

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Date:   Mon, 19 Jun 2017 16:22:29 +0100
From:   Catalin Marinas <catalin.marinas@....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>,
        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>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCHv2 1/3] x86/mm: Provide pmdp_establish() helper

Hi Kirill,

On Thu, Jun 15, 2017 at 05:52:22PM +0300, Kirill A. Shutemov wrote:
> We need an atomic way to setup pmd page table entry, avoiding races with
> CPU setting dirty/accessed bits. This is required to implement
> pmdp_invalidate() that doesn't loose these bits.
> 
> On PAE we have to use cmpxchg8b as we cannot assume what is value of new pmd and
> setting it up half-by-half can expose broken corrupted entry to CPU.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>

I'll look at this from the arm64 perspective. It would be good if we can
have a generic atomic implementation based on cmpxchg64 but I need to
look at the details first.

> +static inline pmd_t pmdp_establish(pmd_t *pmdp, pmd_t pmd)
> +{
> +	pmd_t old;
> +
> +	/*
> +	 * We cannot assume what is value of pmd here, so there's no easy way
> +	 * to set if half by half. We have to fall back to cmpxchg64.
> +	 */
> +	{

BTW, you are missing a "do" here (and it probably compiles just fine
without it, though different behaviour).

> +		old = *pmdp;
> +	} while (cmpxchg64(&pmdp->pmd, old.pmd, pmd.pmd) != old.pmd);
> +
> +	return old;
> +}

-- 
Catalin