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
