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Date:   Thu, 5 Jan 2023 16:29:41 +0100
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Subject: Re: [PATCH 09/46] mm: add MADV_SPLIT to enable HugeTLB HGM
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To:     James Houghton <jthoughton@google.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Peter Xu <peterx@redhat.com>
Cc:     David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230105101844.1893104-1-jthoughton@google.com>
 <20230105101844.1893104-10-jthoughton@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230105101844.1893104-10-jthoughton@google.com>
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On 05.01.23 11:18, James Houghton wrote:
> Issuing ioctl(MADV_SPLIT) on a HugeTLB address range will enable
> HugeTLB HGM. MADV_SPLIT was chosen for the name so that this API can be
> applied to non-HugeTLB memory in the future, if such an application is
> to arise.
> 
> MADV_SPLIT provides several API changes for some syscalls on HugeTLB
> address ranges:
> 1. UFFDIO_CONTINUE is allowed for MAP_SHARED VMAs at PAGE_SIZE
>     alignment.
> 2. read()ing a page fault event from a userfaultfd will yield a
>     PAGE_SIZE-rounded address, instead of a huge-page-size-rounded
>     address (unless UFFD_FEATURE_EXACT_ADDRESS is used).
> 
> There is no way to disable the API changes that come with issuing
> MADV_SPLIT. MADV_COLLAPSE can be used to collapse high-granularity page
> table mappings that come from the extended functionality that comes with
> using MADV_SPLIT.
> 
> For post-copy live migration, the expected use-case is:
> 1. mmap(MAP_SHARED, some_fd) primary mapping
> 2. mmap(MAP_SHARED, some_fd) alias mapping
> 3. MADV_SPLIT the primary mapping
> 4. UFFDIO_REGISTER/etc. the primary mapping
> 5. Copy memory contents into alias mapping and UFFDIO_CONTINUE the
>     corresponding PAGE_SIZE sections in the primary mapping.
> 
> More API changes may be added in the future.
> 
> Signed-off-by: James Houghton <jthoughton@google.com>
> ---
>   arch/alpha/include/uapi/asm/mman.h     |  2 ++
>   arch/mips/include/uapi/asm/mman.h      |  2 ++
>   arch/parisc/include/uapi/asm/mman.h    |  2 ++
>   arch/xtensa/include/uapi/asm/mman.h    |  2 ++
>   include/linux/hugetlb.h                |  2 ++
>   include/uapi/asm-generic/mman-common.h |  2 ++
>   mm/hugetlb.c                           |  3 +--
>   mm/madvise.c                           | 26 ++++++++++++++++++++++++++
>   8 files changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 763929e814e9..7a26f3648b90 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -78,6 +78,8 @@
>   
>   #define MADV_COLLAPSE	25		/* Synchronous hugepage collapse */
>   
> +#define MADV_SPLIT	26		/* Enable hugepage high-granularity APIs */

I think we should make a split more generic, such that it also splits 
(pte-maps) a THP. Has that been discussed?

-- 
Thanks,

David / dhildenb