Re: [PATCH v2 00/46] hugetlb: introduce HugeTLB high-granularity mapping

[David Hildenbrand]

On 22.02.23 21:57, Mina Almasry wrote:
> On Wed, Feb 22, 2023 at 7:49 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 21.02.23 22:46, Mike Kravetz wrote:
> > > On 02/18/23 00:27, James Houghton wrote:
> > > > This series introduces the concept of HugeTLB high-granularity mapping
> > > > (HGM). This series teaches HugeTLB how to map HugeTLB pages at
> > > > high-granularity, similar to how THPs can be PTE-mapped.
> > > >
> > > > Support for HGM in this series is for MAP_SHARED VMAs on x86_64 only. Other
> > > > architectures and (some) support for MAP_PRIVATE will come later.
> > > >
> > > > This series is based on latest mm-unstable (ccd6a73daba9).
> > > >
> > > > Notable changes with this series
> > > > ================================
> > > >
> > > >    - hugetlb_add_file_rmap / hugetlb_remove_rmap are added to handle
> > > >      mapcounting for non-anon hugetlb.
> > > >    - The mapcounting scheme uses subpages' mapcounts for high-granularity
> > > >      mappings, but it does not use subpages_mapcount(). This scheme
> > > >      prevents the HugeTLB VMEMMAP optimization from being used, so it
> > > >      will be improved in a later series.
> > > >    - page_add_file_rmap and page_remove_rmap are updated so they can be
> > > >      used by hugetlb_add_file_rmap / hugetlb_remove_rmap.
> > > >    - MADV_SPLIT has been added to enable the userspace API changes that
> > > >      HGM allows for: high-granularity UFFDIO_CONTINUE (and maybe other
> > > >      changes in the future). MADV_SPLIT does NOT force all the mappings to
> > > >      be PAGE_SIZE.
> > > >    - MADV_COLLAPSE is expanded to include HugeTLB mappings.
> > > >
> > > > Old versions:
> > > > v1: https://lore.kernel.org/linux-mm/20230105101844.1893104-1-jthoughton@xxxxxxxxxx/
> > > > RFC v2: https://lore.kernel.org/linux-mm/20221021163703.3218176-1-jthoughton@xxxxxxxxxx/
> > > > RFC v1: https://lore.kernel.org/linux-mm/20220624173656.2033256-1-jthoughton@xxxxxxxxxx/
> > > >
> > > > Changelog:
> > > > v1 -> v2 (thanks Peter for all your suggestions!):
> > > > - Changed mapcount to be more THP-like, and make HGM incompatible with
> > > >     HVO.
> > > > - HGM is now disabled by default to leave HVO enabled by default.
> > >
> > > I understand the reasoning behind the move to THP-like mapcounting, and the
> > > incompatibility with HVO.  However, I just got to patch 5 and realized either
> > > HGM or HVO will need to be chosen at kernel build time.  That may not be an
> > > issue for cloud providers or others building their own kernels for internal
> > > use.  However, distro kernels will need to pick one option or the other.
> > > Right now, my Fedora desktop has HVO enabled so it would likely not have
> > > HGM enabled.  That is not a big deal for a desktop.
> > >
> > > Just curious, do we have distro kernel users that want to use HGM?
> >
> > Most certainly I would say :)
>
> Is it a blocker to merge in an initial implementation though? Do
> distro kernel users have a pressing need for HVO + HGM used in tandem?

At least RHEL9 seems to include HVO. It's not enabled as default 
(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON not set), but compiled 
in so it can be runtime-enabled. Disabling HVO is not an option IMHO.

Maybe, one could make both features compile-time compatible but 
runtime-mutually exclusive. Or work on a way to make them fully 
compatible right from the start.

--
Thanks,

David / dhildenb