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 16:05:21 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Catalin Marinas <catalin.marinas@....com>,
	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 07/22/2015 03:48 PM, Catalin Marinas wrote:
> 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.

The last time I looked, the instruction to invalidate a single page is
more expensive than the instruction to flush the entire TLB.  We also
don't bother doing ranged flushes _ever_ for hugetlbfs TLB
invalidations, but that was just because the work done around commit
e7b52ffd4 didn't see any benefit.

That said, I can't imagine this will hurt anything.  We also have TLBs
that can mix 2M and 4k pages and I don't think we did back when we put
that code in originally.

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