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

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On 22 July 2015 at 22:39, David Rientjes <rientjes@google.com> wrote:
> 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@arm.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Andrea Arcangeli <aarcange@redhat.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.

You are right, on x86 the tlb_single_page_flush_ceiling seems to be
33, so for an HPAGE_SIZE range the code does a local_flush_tlb()
always. I would say a single page TLB flush is more efficient than a
whole TLB flush but I'm not familiar enough with x86.

Alternatively, I could introduce a flush_tlb_pmd_huge_page (suggested
by Andrea separately) and let the architectures deal with this as they
see fit. The default definition would do a flush_tlb_range(vma,
address, address + HPAGE_SIZE). For arm64, I'll define it as
flush_tlb_page(vma, address).

-- 
Catalin
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[PATCH] mm: Flush the TLB for a single address in a huge page Catalin Marinas <catalin.marinas@arm.com> - 2015-07-22 19:30 +0200
  Re: [PATCH] mm: Flush the TLB for a single address in a huge page Catalin Marinas <catalin.marinas@arm.com> - 2015-07-23 00:50 +0200
    Re: [PATCH] mm: Flush the TLB for a single address in a huge page Dave Hansen <dave.hansen@intel.com> - 2015-07-23 01:10 +0200
      Re: [PATCH] mm: Flush the TLB for a single address in a huge page Andrea Arcangeli <aarcange@redhat.com> - 2015-07-23 16:20 +0200
        Re: [PATCH] mm: Flush the TLB for a single address in a huge page Andrea Arcangeli <aarcange@redhat.com> - 2015-07-23 19:20 +0200

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