Date:   Fri, 8 Dec 2023 11:38:36 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.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>
Subject: Re: [PATCH RFC 00/39] mm/rmap: interface overhaul

On 08/12/2023 11:24, David Hildenbrand wrote:
> 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.

Get's my vote!

> 
> Thanks!
> 

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