linux-kernel - Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

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Message-ID: <08de71f6-f049-4c3b-977e-f658f85cd734@arm.com>
Date: Tue, 7 May 2024 13:36:45 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Yang Shi <shy828301@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>,
 Yang Shi <yang@...amperecomputing.com>, riel@...riel.com, cl@...ux.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Ze Zuo <zuoze1@...wei.com>
Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP
 boundaries

On 07/05/2024 12:42, David Hildenbrand wrote:
> On 07.05.24 13:34, David Hildenbrand wrote:
>> On 07.05.24 13:26, Ryan Roberts wrote:
>>> On 07/05/2024 12:14, Ryan Roberts wrote:
>>>> On 07/05/2024 12:13, David Hildenbrand wrote:
>>>>>
>>>>>> https://github.com/intel/lmbench/blob/master/src/lat_mem_rd.c#L95
>>>>>>
>>>>>>> suggest. If you want to try something semi-randomly; it might be useful
>>>>>>> to rule
>>>>>>> out the arm64 contpte feature. I don't see how that would be interacting
>>>>>>> here if
>>>>>>> mTHP is disabled (is it?). But its new for 6.9 and arm64 only. Disable with
>>>>>>> ARM64_CONTPTE (needs EXPERT) at compile time.
>>>>>> I don't enabled mTHP, so it should be not related about ARM64_CONTPTE,
>>>>>> but will have a try.
>>>>>
>>>>> cont-pte can get active if we're just lucky when allocating pages in the right
>>>>> order, correct Ryan?
>>>>
>>>> No it shouldn't do; it requires the pages to be in the same folio.
>>
>> Ah, my memory comes back. That's also important for folio_pte_batch() to
>> currently work as expected I think. We could change that, though, and
>> let cont-pte batch across folios.
> 
> Thinking about it (and trying to refresh my memories), access/dirty bits might
> be why we don't want to do that.

Yes correct; we only get a single access/dirty bit for the whole contpte block.
So can't honour the core kernel's tracking requirements when the pages are not
part of a single folio.