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Date: Tue, 7 May 2024 13:36:45 +0100
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Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP
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To: David Hildenbrand <david@redhat.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>, Yang Shi <shy828301@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>,
 Yang Shi <yang@os.amperecomputing.com>, riel@surriel.com, cl@linux.com,
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From: Ryan Roberts <ryan.roberts@arm.com>
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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.