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

[David Hildenbrand]

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 :)

--
Thanks,

David / dhildenb