Re: [PATCH RFC 00/39] mm/rmap: interface overhaul

[David Hildenbrand <david@redhat.com>]

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.

> 
>>
>> 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.
> 

I think having "folio" in there looks cleaner and more consistent to 
other functions.

I tend to like "rmap_dup_file_folio_[pte|ptes|pmd]()", because then we 
have "file folio" and "anon folio" as one word.

But then I wonder about the hugetlb part. Maybe simply 
"hugetlb_rmap_remove_folio()" etc.

Having the "hugetlb_" prefix at the beginning feels like the right thing 
to do, looking at orher hugetlb special-handlings.

But I'll wait a bit until I go crazy on renaming :)

> 
>>
>>>
>>> I guess reading the patches will tell me, but what's the point of "ptes"? Surely
>>> you're either mapping at pte or pmd level, and the number of pages is determined
>>> by the folio size? (or presumably nr param passed in)
>>
>> It's really (currently) one function to handle 1 vs. multiple PTEs. For example:
>>
>> void folio_remove_rmap_ptes(struct folio *, struct page *, unsigned int nr,
>>          struct vm_area_struct *);
>> #define folio_remove_rmap_pte(folio, page, vma) \
>>      folio_remove_rmap_ptes(folio, page, 1, vma)
>> void folio_remove_rmap_pmd(struct folio *, struct page *,
>>          struct vm_area_struct *);
> 
> Yeah now that I've looked at the series, this makes sense. "ptes" was originally
> making me think of contpte, but I suspect I'll be the only one with that
> association :)

Ah, yes :)

-- 
Cheers,

David / dhildenb