Re: [PATCH RFC 06/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/11/2023 19:46, Peter Xu wrote:
> On Thu, Nov 23, 2023 at 07:11:19PM +0000, Ryan Roberts wrote:
>> Hi,
>>
>> I'm not sure I've 100% understood the crossover between this series and my work
>> to support arm64's contpte mappings generally for anonymous and file-backed memory.
> 
> No worry, there's no confliction.  If you worked on that it's only be
> something nice on top.  Also, I'm curious if you have performance numbers,

I have perf numbers for high level use cases (kernel compilation and Speedometer
Java Script benchmarks) at
https://lore.kernel.org/linux-arm-kernel/20230622144210.2623299-1-ryan.roberts@arm.com/

I don't have any micro-benchmarks for GUP though, if that's your question. Is
there an easy-to-use test I can run to get some numbers? I'd be happy to try it out.

> because I'm going to do some test for hugetlb cont_ptes (which is only the
> current plan), and if you got those it'll be a great baseline for me,
> because it should be similar in you case even though the goal is slightly
> different.
> 
>>
>> My approach is to transparently use contpte mappings when core-mm request pte
>> mappings that meet the requirements; and its all based around intercepting the
>> normal (non-hugetlb) helpers (e.g. set_ptes(), ptep_get() and friends). There is
>> no semantic change to the core-mm. See [1]. It relies on 1) the page cache using
>> large folios and 2) my "small-sized THP" series which starts using arbitrary
>> sized large folios for anonymous memory [2].
>>
>> If I've understood this conversation correctly there is an object called hugepd,
>> which today is only supported by powerpc, but which could allow the core-mm to
>> control the mapping granularity? I can see some value in exposing that control
>> to core-mm in the (very) long term.
> 
> For me it's needed immediately, because hugetlb_follow_page_mask() will be
> gone after the last patch.
> 
>>
>> [1] https://lore.kernel.org/all/20231115163018.1303287-1-ryan.roberts@arm.com/
>> [2] https://lore.kernel.org/linux-mm/20231115132734.931023-1-ryan.roberts@arm.com/
> 
> AFAICT you haven't yet worked on gup then, after I glimpsed the above
> series.

No, I haven't touched GUP at all. The approach is fully inside the arm64 arch
code (except 1 patch to core-mm which enables an optimization). So as far as GUP
and the rest of the core-mm is concerned, there are still only page-sized ptes
and they can all be iterated over and accessed as normal.

> 
> It's a matter of whether one follow_page_mask() call can fetch more than
> one page* for a cont_pte entry on aarch64 for a large non-hugetlb folio
> (and if this series lands, it'll be the same to hugetlb or non-hugetlb).
> Now the current code can only fetch one page I think.
> 
> Thanks,
>