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

[David Hildenbrand]

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