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

[David Hildenbrand <david@redhat.com>]

On 22.02.23 21:57, Mina Almasry wrote:
> On Wed, Feb 22, 2023 at 7:49 AM David Hildenbrand <david@redhat.com> 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@google.com/
>>>> RFC v2: https://lore.kernel.org/linux-mm/20221021163703.3218176-1-jthoughton@google.com/
>>>> RFC v1: https://lore.kernel.org/linux-mm/20220624173656.2033256-1-jthoughton@google.com/
>>>>
>>>> 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