linux-kernel - Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge

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Date:   Mon, 25 Oct 2021 12:52:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: Re: [PATCH v2 2/5] mm: avoid unnecessary flush on change_huge_pmd()

On Thu, Oct 21, 2021 at 05:21:09AM -0700, Nadav Amit wrote:
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 448cd01eb3ec..18c3366f8f4d 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1146,6 +1146,14 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
>  	}
>  }
>  #endif
> +
> +#define __HAVE_ARCH_PMDP_INVALIDATE_AD
> +static inline pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma,
> +				       unsigned long address, pmd_t *pmdp)
> +{
> +	return pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));

Did this want to be something like:

	pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
	if (cpu_feature_enabled(X86_BUG_PTE_LEAK))
		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
	return old;

instead?

> +}
> +
>  /*
>   * Page table pages are page-aligned.  The lower half of the top
>   * level is used for userspace and the top half for the kernel.

> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e5ea5f775d5c..435da011b1a2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1795,10 +1795,11 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  	 * The race makes MADV_DONTNEED miss the huge pmd and don't clear it
>  	 * which may break userspace.
>  	 *
> -	 * pmdp_invalidate() is required to make sure we don't miss
> -	 * dirty/young flags set by hardware.
> +	 * pmdp_invalidate_ad() is required to make sure we don't miss
> +	 * dirty/young flags (which are also known as access/dirty) cannot be
> +	 * further modifeid by the hardware.

"modified", I think is the more common spelling.

>  	 */
> -	entry = pmdp_invalidate(vma, addr, pmd);
> +	entry = pmdp_invalidate_ad(vma, addr, pmd);
>  
>  	entry = pmd_modify(entry, newprot);
>  	if (preserve_write)
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 4e640baf9794..b0ce6c7391bf 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -200,6 +200,14 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  }
>  #endif
>  
> +#ifndef __HAVE_ARCH_PMDP_INVALIDATE_AD

/*
 * Does this deserve a comment to explain the intended difference vs
 * pmdp_invalidate() ?
 */

> +pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,
> +			 pmd_t *pmdp)
> +{
> +	return pmdp_invalidate(vma, address, pmdp);
> +}
> +#endif
> +
>  #ifndef pmdp_collapse_flush
>  pmd_t pmdp_collapse_flush(struct vm_area_struct *vma, unsigned long address,
>  			  pmd_t *pmdp)
> -- 
> 2.25.1
>