Re: [PATCH v2 09/46] mm: add MADV_SPLIT to enable HugeTLB HGM

[Mina Almasry <almasrymina@google.com>]

On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@google.com> 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.
>

So is a hugetlb page or VMA that has been MADV_SPLIT + MADV_COLLAPSE
distinct from a hugetlb page or vma that has not been? I thought
COLLAPSE would reverse the effects on SPLIT completely.

> 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.
>

Huh, so MADV_SPLIT doesn't actually split an existing PMD mapping into
high granularity mappings. Instead it says that future mappings may be
high granularity? I assume they may not even be high granularity, like
if the alias mapping faulted in a full hugetlb page (without
UFFDIO_CONTINUE) that page would be regular mapped not high
granularity mapped.

This may be bikeshedding but I do think a clearer name is warranted.
Maybe MADV_MAY_SPLIT or something.

> More API changes may be added in the future.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
>
> 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 */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index c6e1fc77c996..f8a74a3a0928 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -105,6 +105,8 @@
>
>  #define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
>
> +#define MADV_SPLIT     26              /* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index 68c44f99bc93..a6dc6a56c941 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -72,6 +72,8 @@
>
>  #define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
>
> +#define MADV_SPLIT     74              /* Enable hugepage high-granularity APIs */
> +
>  #define MADV_HWPOISON     100          /* poison a page for testing */
>  #define MADV_SOFT_OFFLINE 101          /* soft offline page for testing */
>
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index 1ff0c858544f..f98a77c430a9 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -113,6 +113,8 @@
>
>  #define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
>
> +#define MADV_SPLIT     26              /* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 6ce1f1ceb432..996e8ded092f 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -79,6 +79,8 @@
>
>  #define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
>
> +#define MADV_SPLIT     26              /* Enable hugepage high-granularity APIs */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index c2202f51e9dd..8c004c678262 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1006,6 +1006,28 @@ static long madvise_remove(struct vm_area_struct *vma,
>         return error;
>  }
>
> +static int madvise_split(struct vm_area_struct *vma,
> +                        unsigned long *new_flags)
> +{
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +       if (!is_vm_hugetlb_page(vma) || !hugetlb_hgm_eligible(vma))
> +               return -EINVAL;
> +
> +       /*
> +        * PMD sharing doesn't work with HGM. If this MADV_SPLIT is on part
> +        * of a VMA, then we will split the VMA. Here, we're unsharing before
> +        * splitting because it's simpler, although we may be unsharing more
> +        * than we need.
> +        */
> +       hugetlb_unshare_all_pmds(vma);
> +
> +       *new_flags |= VM_HUGETLB_HGM;
> +       return 0;
> +#else
> +       return -EINVAL;
> +#endif
> +}
> +
>  /*
>   * Apply an madvise behavior to a region of a vma.  madvise_update_vma
>   * will handle splitting a vm area into separate areas, each area with its own
> @@ -1084,6 +1106,11 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>                 break;
>         case MADV_COLLAPSE:
>                 return madvise_collapse(vma, prev, start, end);
> +       case MADV_SPLIT:
> +               error = madvise_split(vma, &new_flags);
> +               if (error)
> +                       goto out;
> +               break;
>         }
>
>         anon_name = anon_vma_name(vma);
> @@ -1178,6 +1205,9 @@ madvise_behavior_valid(int behavior)
>         case MADV_HUGEPAGE:
>         case MADV_NOHUGEPAGE:
>         case MADV_COLLAPSE:
> +#endif
> +#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> +       case MADV_SPLIT:
>  #endif
>         case MADV_DONTDUMP:
>         case MADV_DODUMP:
> @@ -1368,6 +1398,8 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
>   *             transparent huge pages so the existing pages will not be
>   *             coalesced into THP and new pages will not be allocated as THP.
>   *  MADV_COLLAPSE - synchronously coalesce pages into new THP.
> + *  MADV_SPLIT - allow HugeTLB pages to be mapped at PAGE_SIZE. This allows
> + *             UFFDIO_CONTINUE to accept PAGE_SIZE-aligned regions.
>   *  MADV_DONTDUMP - the application wants to prevent pages in the given range
>   *             from being included in its core dump.
>   *  MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump.
> --
> 2.39.2.637.g21b0678d19-goog
>