Re: [RFC PATCH 0/1] Add hugetlbfs support to memfd_create()

[Michal Hocko]

Hi,
I am one foot out of office and will be offline for two days so I
didn't get to review the patch yet but this information is an useful
information about the usecase that should be in the patch directly for
future reference.

On Mon 07-08-17 16:47:51, Mike Kravetz wrote:
> This patch came out of discussions in this e-mail thread [1].
> 
> The Oracle JVM team is developing a new garbage collection model.  This
> new model requires multiple mappings of the same anonymous memory.  One
> straight forward way to accomplish this is with memfd_create.  They can
> use the returned fd to create multiple mappings of the same memory.
> 
> The JVM today has an option to use (static hugetlb) huge pages.  If this
> option is specified, they would like to use the same garbage collection
> model requiring multiple mappings to the same memory.  Using hugetlbfs,
> it is possible to explicitly mount a filesystem and specify file paths
> in order to get an fd that can be used for multiple mappings.  However,
> this introduces additional system admin work and coordination.
> 
> Ideally they would like to get a hugetlbfs fd without requiring explicit
> mounting of a filesystem.   Today, mmap and shmget can make use of
> hugetlbfs without explicitly mounting a filesystem.  The patch adds this
> functionality to hugetlbfs.
> 
> A new flag MFD_HUGETLB is introduced to request a hugetlbfs file.  Like
> other system calls where hugetlb can be requested, the huge page size
> can be encoded in the flags argument is the non-default huge page size
> is desired.  hugetlbfs does not support sealing operations, therefore
> specifying MFD_ALLOW_SEALING with MFD_HUGETLB will result in EINVAL.
> 
> Of course, the memfd_man page would need updating if this type of
> functionality moves forward.
> 
> [1] https://lkml.org/lkml/2017/7/6/564
> 
> Mike Kravetz (1):
>   mm/shmem: add hugetlbfs support to memfd_create()
> 
>  include/uapi/linux/memfd.h | 24 ++++++++++++++++++++++++
>  mm/shmem.c                 | 37 +++++++++++++++++++++++++++++++------
>  2 files changed, 55 insertions(+), 6 deletions(-)
> 
> -- 
> 2.7.5

-- 
Michal Hocko
SUSE Labs