Re: [PATCH 2/3] Add MAP_LARGEPAGE for mmaping pseudo-anonymous huge page regions

[Pekka Enberg]

Hi Eric,

On Wed, Aug 12, 2009 at 1:13 AM, Eric B Munson<ebmunson@xxxxxxxxxx> wrote:
> This patch adds a flag for mmap that will be used to request a huge
> page region that will look like anonymous memory to user space.  This
> is accomplished by using a file on the internal vfsmount.  MAP_LARGEPAGE
> is a modifier of MAP_ANONYMOUS and so must be specified with it.  The
> region will behave the same as a MAP_ANONYMOUS region using small pages.
>
> Signed-off-by: Eric B Munson <ebmunson@xxxxxxxxxx>

I would love to see something like this in the kernel. Huge pages are
useful for garbage collection and JIT text in the userspace but
unfortunately obtaining them is a real PITA at the moment.

Is there any way to drop the
CAP_IPC_LOCK/in_group_p(hugetlbfs_shm_group) requirement, btw? That
would make huge pages even more accessible to user-space virtual
machines.

                        Pekka

> ---
>  include/asm-generic/mman-common.h |    1 +
>  include/linux/hugetlb.h           |    7 +++++++
>  mm/mmap.c                         |   16 ++++++++++++++++
>  3 files changed, 24 insertions(+), 0 deletions(-)
>
> diff --git a/include/asm-generic/mman-common.h b/include/asm-generic/mman-common.h
> index 3b69ad3..60b6be7 100644
> --- a/include/asm-generic/mman-common.h
> +++ b/include/asm-generic/mman-common.h
> @@ -19,6 +19,7 @@
>  #define MAP_TYPE       0x0f            /* Mask for type of mapping */
>  #define MAP_FIXED      0x10            /* Interpret addr exactly */
>  #define MAP_ANONYMOUS  0x20            /* don't use a file */
> +#define MAP_LARGEPAGE  0x40            /* create a large page mapping */
>
>  #define MS_ASYNC       1               /* sync memory asynchronously */
>  #define MS_INVALIDATE  2               /* invalidate the caches */
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 78b6ddf..b84361c 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -109,12 +109,19 @@ static inline void hugetlb_report_meminfo(struct seq_file *m)
>
>  #endif /* !CONFIG_HUGETLB_PAGE */
>
> +#define HUGETLB_ANON_FILE "anon_hugepage"
> +
>  enum {
>        /*
>         * The file will be used as an shm file so shmfs accounting rules
>         * apply
>         */
>        HUGETLB_SHMFS_INODE     = 0x01,
> +       /*
> +        * The file is being created on the internal vfs mount and shmfs
> +        * accounting rules do not apply
> +        */
> +       HUGETLB_ANONHUGE_INODE  = 0x02,
>  };
>
>  #ifdef CONFIG_HUGETLBFS
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 34579b2..c2c729a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -29,6 +29,7 @@
>  #include <linux/rmap.h>
>  #include <linux/mmu_notifier.h>
>  #include <linux/perf_counter.h>
> +#include <linux/hugetlb.h>
>
>  #include <asm/uaccess.h>
>  #include <asm/cacheflush.h>
> @@ -954,6 +955,21 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
>        if (mm->map_count > sysctl_max_map_count)
>                return -ENOMEM;
>
> +       if (flags & MAP_LARGEPAGE) {
> +               if (file)
> +                       return -EINVAL;
> +
> +               /*
> +                * VM_NORESERVE is used because the reservations will be
> +                * taken when vm_ops->mmap() is called
> +                */
> +               len = ALIGN(len, huge_page_size(&default_hstate));
> +               file = hugetlb_file_setup(HUGETLB_ANON_FILE, len, VM_NORESERVE,
> +                                               HUGETLB_ANONHUGE_INODE);
> +               if (IS_ERR(file))
> +                       return -ENOMEM;
> +       }
> +
>        /* Obtain the address to map to. we verify (or select) it and ensure
>         * that it represents a valid section of the address space.
>         */
> --
> 1.6.3.2
>
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