linux-kernel - Re: [PATCH] mm: Flush the TLB for a single address in a huge page

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Date:	Wed, 22 Jul 2015 14:39:34 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Catalin Marinas <catalin.marinas@....com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH] mm: Flush the TLB for a single address in a huge page

On Wed, 22 Jul 2015, Catalin Marinas wrote:

> When the page table entry is a huge page (and not a table), there is no
> need to flush the TLB by range. This patch changes flush_tlb_range() to
> flush_tlb_page() in functions where we know the pmd entry is a huge
> page.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> ---
> 
> Hi,
> 
> That's just a minor improvement but it saves iterating over each small
> page in a huge page when a single TLB entry is used (we already have a
> similar assumption in __tlb_adjust_range).
> 

For x86 smp, this seems to mean the difference between unconditional 
flush_tlb_page() and local_flush_tlb() due to 
tlb_single_page_flush_ceiling, so I don't think this just removes the 
iteration.

> Thanks.
> 
>  mm/pgtable-generic.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 6b674e00153c..ff17eca26211 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -67,7 +67,7 @@ int pmdp_set_access_flags(struct vm_area_struct *vma,
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  	if (changed) {
>  		set_pmd_at(vma->vm_mm, address, pmdp, entry);
> -		flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +		flush_tlb_page(vma, address);
>  	}
>  	return changed;
>  #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> @@ -101,7 +101,7 @@ int pmdp_clear_flush_young(struct vm_area_struct *vma,
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  	young = pmdp_test_and_clear_young(vma, address, pmdp);
>  	if (young)
> -		flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +		flush_tlb_page(vma, address);
>  	return young;
>  }
>  #endif
> @@ -128,7 +128,7 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  	VM_BUG_ON(!pmd_trans_huge(*pmdp));
>  	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
> -	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +	flush_tlb_page(vma, address);
>  	return pmd;
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> @@ -143,7 +143,7 @@ void pmdp_splitting_flush(struct vm_area_struct *vma, unsigned long address,
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  	set_pmd_at(vma->vm_mm, address, pmdp, pmd);
>  	/* tlb flush only to serialize against gup-fast */
> -	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +	flush_tlb_page(vma, address);
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #endif
> @@ -195,7 +195,7 @@ void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>  {
>  	pmd_t entry = *pmdp;
>  	set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry));
> -	flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +	flush_tlb_page(vma, address);
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #endif
--
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