Re: [patch] memfd_create.2: Add description of MFD_HUGETLB (hugetlbfs) support

[Mike Kravetz]

CC: linux-mm

On 09/15/2017 02:43 PM, Mike Kravetz wrote:
> hugetlbfs support for memfd_create was recently merged by Linus and
> should be in the Linux 4.14 release.  To request hugetlbfs support
> a new memfd_create flag (MFD_HUGETLB) was added.
> 
> This patch documents the following commit:
> 
> commit 749df87bd7bee5a79cef073f5d032ddb2b211de8
> Author: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Date:   Wed Sep 6 16:24:16 2017 -0700
> 
>     mm/shmem: add hugetlbfs support to memfd_create()
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  man2/memfd_create.2 | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/man2/memfd_create.2 b/man2/memfd_create.2
> index 4dfd1bb2d..b61254bb8 100644
> --- a/man2/memfd_create.2
> +++ b/man2/memfd_create.2
> @@ -100,6 +100,33 @@ If this flag is not set, the initial set of seals will be
>  meaning that no other seals can be set on the file.
>  .\" FIXME Why is the MFD_ALLOW_SEALING behavior not simply the default?
>  .\" Is it worth adding some text explaining this?
> +.TP
> +.BR MFD_HUGETLB " (since Linux 4.14)"
> +The anonymous file will be created in the hugetlbfs filesystem using
> +huge pages.  See the Linux kernel source file
> +.I Documentation/vm/hugetlbpage.txt
> +for more information about hugetlbfs.  The hugetlbfs filesystem does
> +not support file sealing operations.  Therefore, specifying both
> +.B MFD_HUGETLB
> +and
> +.B MFD_ALLOW_SEALING
> +will result in an error
> +.RB (EINVAL)
> +being returned.
> +
> +.TP
> +.BR MFD_HUGE_2MB ", " MFD_HUGE_1GB ", " "..."
> +Used in conjunction with
> +.B MFD_HUGETLB
> +to select alternative hugetlb page sizes (respectively, 2 MB, 1 GB, ...)
> +on systems that support multiple hugetlb page sizes.  Definitions for known
> +huge page sizes are included in the header file
> +.I <sys/memfd.h>.
> +
> +For details on encoding huge page sizes not included in the header file,
> +see the discussion of the similarly named constants in
> +.BR mmap (2).
> +
>  .PP
>  Unused bits in
>  .I flags
>