linux-kernel - Re: [PATCH 5/6] x86: mm: new tunable for single vs full TLB flush

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Date:	Thu, 24 Apr 2014 10:25:52 -0700
From:	Dave Hansen <dave@...1.net>
To:	Mel Gorman <mgorman@...e.de>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
	ak@...ux.intel.com, riel@...hat.com, alex.shi@...aro.org,
	dave.hansen@...ux.intel.com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 5/6] x86: mm: new tunable for single vs full TLB flush

On 04/24/2014 03:37 AM, Mel Gorman wrote:
> On Mon, Apr 21, 2014 at 11:24:26AM -0700, Dave Hansen wrote:
>> +This will cause us to do the global flush for more cases.
>> +Lowering it to 0 will disable the use of the individual flushes.
>> +Setting it to 1 is a very conservative setting and it should
>> +never need to be 0 under normal circumstances.
>> +
>> +Despite the fact that a single individual flush on x86 is
>> +guaranteed to flush a full 2MB, hugetlbfs always uses the full
>> +flushes.  THP is treated exactly the same as normal memory.
>> +
> 
> You are the second person that told me this and I felt the manual was
> unclear on this subject. I was told that it might be a documentation bug
> but because this discussion was in a bar I completely failed to follow up
> on it. Specifically this part in 4.10.2.3 caused me problems when I last
> looked at the area.
<snip>

My understanding comes from "4.10.4.2 Recommended Invalidation":

	• If software modifies a paging-structure entry that identifies
	the final page frame for a page number (either a PTE or a
	paging-structure entry in which the PS flag is 1), it should
	execute INVLPG for any linear address with a page number whose
	translation uses that PTE. 2

and especially the footnote:

	2. One execution of INVLPG is sufficient even for a page with
	size greater than 4 KBytes.

I do agree that it's ambiguous at best.  I'll go see if anybody cares to
update that bit.

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