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>
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-googOn 02/18/23 00:27, 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>
>
> 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);
I think we should just unshare the (appropriately aligned) range within the
vma that is the target of MADV_SPLIT. No need to unshare the entire 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;
Not a huge deal, but if one passes an invalid range (such as not huge page
size aligned) to MADV_SPLIT, then we will not notice the error until
later in madvise_update_vma() when the vma split fails. By then, we will
have unshared all pmds in the entire vma (or just the range if you agree
with my suggestion above).
--
Mike Kravetz
> + 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
>
On Fri, Feb 24, 2023 at 3:25 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 02/18/23 00:27, 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>
> >
> > 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);
>
> I think we should just unshare the (appropriately aligned) range within the
> vma that is the target of MADV_SPLIT. No need to unshare the entire vma.
Right I can do that, and I can check for appropriate alignment here
(else fail with -EINVAL).
>
> > +
> > + *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;
>
> Not a huge deal, but if one passes an invalid range (such as not huge page
> size aligned) to MADV_SPLIT, then we will not notice the error until
> later in madvise_update_vma() when the vma split fails. By then, we will
> have unshared all pmds in the entire vma (or just the range if you agree
> with my suggestion above).
Good point. I'll fix this for v3. :) Thanks Mike.
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
>
On Fri, Feb 17, 2023 at 5:58 PM Mina Almasry <almasrymina@google.com> wrote: > > 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. Right now, MADV_COLLAPSE does *not* completely undo the effects of an MADV_SPLIT. The API changes that come from MADV_SPLIT aren't undone with an MADV_COLLAPSE. > > > 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. MADV_SPLIT just means "userspace is aware that they are able to start mapping HugeTLB pages at high-granularity". Right now the only way to get high-granularity mappings is with UFFDIO_CONTINUE, but there may be other ways in the future. As of this series, if you MADV_SPLIT a HugeTLB VMA and you aren't using userfaultfd minor faults, it's basically a no-op. The mappings that are created will still be huge. I could change this, but I don't really see a reason to right now. > > This may be bikeshedding but I do think a clearer name is warranted. > Maybe MADV_MAY_SPLIT or something. I agree -- MADV_MAY_SPLIT more accurately describes the HugeTLB functionality. I really don't mind what the MADV is called. I think enabling the high-granularity userfaultfd bits with a userfaultfd feature[1] worked reasonably well. There is some API discussion in that thread[1]. [1]: https://lore.kernel.org/linux-mm/20221021163703.3218176-34-jthoughton@google.com/
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