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

[David Hildenbrand <david@redhat.com>]

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 :)

-- 
Thanks,

David / dhildenb