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

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Message-ID: <CAHkRjk7=VMG63VfZdWbZqYu8FOa9M+54Mmdro661E2zt3WToog@mail.gmail.com>
Date:	Wed, 22 Jul 2015 23:48:42 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <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 22 July 2015 at 22:39, David Rientjes <rientjes@...gle.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@....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.

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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