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

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Date:   Thu, 23 Feb 2023 17:17:31 +0100
From:   David Hildenbrand <david@...hat.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Mina Almasry <almasrymina@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Zach O'Keefe <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Frank van der Linden <fvdl@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/46] hugetlb: introduce HugeTLB high-granularity
 mapping

On 23.02.23 16:53, James Houghton wrote:
> On Thu, Feb 23, 2023 at 1:07 AM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 22.02.23 21:57, Mina Almasry wrote:
>>> On Wed, Feb 22, 2023 at 7:49 AM David Hildenbrand <david@...hat.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 :)
> 
> I'm not sure. Maybe distros want the hwpoison benefits HGM provides?
> But that's not implemented in this series.

 From what I can tell, HGM helps to improve live migration of VMs with 
gigantic pages. That sounds like a good reason why distros (that support 
virtualization) might want it independent of hwpoison changes.

-- 
Thanks,

David / dhildenb