linux-kernel - Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD

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Message-ID: <alpine.DEB.2.21.1908201106260.2223@nanos.tec.linutronix.de>
Date:   Tue, 20 Aug 2019 11:12:13 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Song Liu <songliubraving@...com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com, stable@...r.kernel.org,
        Joerg Roedel <jroedel@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr
 by PUD_SIZE

On Tue, 20 Aug 2019, Song Liu wrote:

> pti_clone_pgtable() increases addr by PUD_SIZE for pud_none(*pud) case.
> This is not accurate because addr may not be PUD_SIZE aligned.

You fail to explain how this happened. The code before the 32bit support
did always increase by PMD_SIZE. The 32bit support broke that.
 
> In our x86_64 kernel, pti_clone_pgtable() fails to clone 7 PMDs because
> of this issuse, including PMD for the irq entry table. For a memcache
> like workload, this introduces about 4.5x more iTLB-load and about 2.5x
> more iTLB-load-misses on a Skylake CPU.

This information is largely irrelevant. What matters is the fact that this
got broken and incorrectly forwards the address by PUD_SIZE which is wrong
if address is not PUD_SIZE aligned.

> This patch fixes this issue by adding PMD_SIZE to addr for pud_none()
> case.

  git grep 'This patch' Documentation/process/submitting-patches.rst

> Cc: stable@...r.kernel.org # v4.19+
> Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
> Signed-off-by: Song Liu <songliubraving@...com>
> Cc: Joerg Roedel <jroedel@...e.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  arch/x86/mm/pti.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> index b196524759ec..5a67c3015f59 100644
> --- a/arch/x86/mm/pti.c
> +++ b/arch/x86/mm/pti.c
> @@ -330,7 +330,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
>  
>  		pud = pud_offset(p4d, addr);
>  		if (pud_none(*pud)) {
> -			addr += PUD_SIZE;
> +			addr += PMD_SIZE;

The right fix is to skip forward to the next PUD boundary instead of doing
this in a loop with PMD_SIZE increments.

Thanks,

	tglx