Date:   Fri, 8 Dec 2023 12:24:11 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Peter Xu <peterx@...hat.com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>
Subject: Re: [PATCH RFC 00/39] mm/rmap: interface overhaul

On 05.12.23 14:49, Ryan Roberts wrote:
> On 05/12/2023 13:39, David Hildenbrand wrote:
>> On 05.12.23 14:31, Ryan Roberts wrote:
>>> On 05/12/2023 09:56, David Hildenbrand wrote:
>>>>>>
>>>>>> Ryan has series where we would make use of folio_remove_rmap_ptes() [1]
>>>>>> -- he carries his own batching variant right now -- and
>>>>>> folio_try_dup_anon_rmap_ptes()/folio_dup_file_rmap_ptes() [2].
>>>>>
>>>>> Note that the contpte series at [2] has a new patch in v3 (patch 2), which
>>>>> could
>>>>> benefit from folio_remove_rmap_ptes() or equivalent. My plan was to revive [1]
>>>>> on top of [2] once it is merged.
>>>>>
>>>>>>
>>>>>> There is some overlap with both series (and some other work, like
>>>>>> multi-size THP [3]), so that will need some coordination, and likely a
>>>>>> stepwise inclusion.
>>>>>
>>>>> Selfishly, I'd really like to get my stuff merged as soon as there is no
>>>>> technical reason not to. I'd prefer not to add this as a dependency if we can
>>>>> help it.
>>>>
>>>> It's easy to rework either series on top of each other. The mTHP series has
>>>> highest priority,
>>>> no question, that will go in first.
>>>
>>> Music to my ears! It would be great to either get a reviewed-by or feedback on
>>> why not, for the key 2 patches in that series (3 & 4) and also your opinion on
>>> whether we need to wait for compaction to land (see cover letter). It would be
>>> great to get this into linux-next ASAP IMHO.
>>
>> On it :)
>>
>>>
>>>>
>>>> Regarding the contpte, I think it needs more work. Especially, as raised, to not
>>>> degrade
>>>> order-0 performance. Maybe we won't make the next merge window (and you already
>>>> predicated
>>>> that in some cover letter :P ). Let's see.
>>>
>>> Yeah that's ok. I'll do the work to fix the order-0 perf. And also do the same
>>> for patch 2 in that series - would also be really helpful if you had a chance to
>>> look at patch 2 - its new for v3.
>>
>> I only skimmed over it, but it seems to go into the direction we'll need.
>> Keeping order-0 performance unharmed should have highest priority. Hopefully my
>> microbenchmarks are helpful.
> 
> Yes absolutely - are you able to share them??
> 
>>
>>>
>>>>
>>>> But again, the conflicts are all trivial, so I'll happily rebase on top of
>>>> whatever is
>>>> in mm-unstable. Or move the relevant rework to the front so you can just carry
>>>> them/base on them. (the batched variants for dup do make the contpte code much
>>>> easier)
>>>
>>> So perhaps we should aim for mTHP, then this, then contpte last, benefiting from
>>> the batching.
>>
>> Yeah. And again, I don't care too much if I have to rebase on top of your work
>> if this here takes longer. It's all a fairly trivial conversion.
>>
>>>>
>>>> [...]
>>>>
>>>>>>
>>>>>>
>>>>>> New (extended) hugetlb interface that operate on entire folio:
>>>>>>     * hugetlb_add_new_anon_rmap() -> Already existed
>>>>>>     * hugetlb_add_anon_rmap() -> Already existed
>>>>>>     * hugetlb_try_dup_anon_rmap()
>>>>>>     * hugetlb_try_share_anon_rmap()
>>>>>>     * hugetlb_add_file_rmap()
>>>>>>     * hugetlb_remove_rmap()
>>>>>>
>>>>>> New "ordinary" interface for small folios / THP::
>>>>>>     * folio_add_new_anon_rmap() -> Already existed
>>>>>>     * folio_add_anon_rmap_[pte|ptes|pmd]()
>>>>>>     * folio_try_dup_anon_rmap_[pte|ptes|pmd]()
>>>>>>     * folio_try_share_anon_rmap_[pte|pmd]()
>>>>>>     * folio_add_file_rmap_[pte|ptes|pmd]()
>>>>>>     * folio_dup_file_rmap_[pte|ptes|pmd]()
>>>>>>     * folio_remove_rmap_[pte|ptes|pmd]()
>>>>>
>>>>> I'm not sure if there are official guidelines, but personally if we are
>>>>> reworking the API, I'd take the opportunity to move "rmap" to the front of the
>>>>> name, rather than having it burried in the middle as it is for some of these:
>>>>>
>>>>> rmap_hugetlb_*()
>>>>>
>>>>> rmap_folio_*()
>>>>
>>>> No strong opinion. But we might want slightly different names then. For example,
>>>> it's "bio_add_folio" and not "bio_folio_add":
>>>>
>>>>
>>>> rmap_add_new_anon_hugetlb()
>>>> rmap_add_anon_hugetlb()
>>>> ...
>>>> rmap_remove_hugetlb()
>>>>
>>>>
>>>> rmap_add_new_anon_folio()
>>>> rmap_add_anon_folio_[pte|ptes|pmd]()
>>>> ...
>>>> rmap_dup_file_folio_[pte|ptes|pmd]()
>>>> rmap_remove_folio_[pte|ptes|pmd]()
>>>>
>>>> Thoughts?
>>>
>>> Having now reviewed your series, I have a less strong opinion, perhaps it's
>>> actually best with your original names; "folio" is actually the subject after
>>> all; it's the thing being operated on.

So far I sticked to the original names used in this RFC. I'm testing a 
new series that is based on current mm/unstable (especially, mTHP) and 
contains all changes discussed here.

If I don't here anything else, I'll send that out as v1 on Monday.

Thanks!

-- 
Cheers,

David / dhildenb

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