Re: [PATCH 07/16] memfd: memfd_create(name, MFD_HUGEPAGE) for shmem huge pages

On Fri, Jul 30, 2021 at 12:45:49AM -0700, Hugh Dickins wrote:

Commit 749df87bd7be ("mm/shmem: add hugetlbfs support to memfd_create()")
in 4.14 added the MFD_HUGETLB flag to memfd_create(), to use hugetlbfs
pages instead of tmpfs pages: now add the MFD_HUGEPAGE flag, to use tmpfs
Transparent Huge Pages when they can be allocated (flag named to follow
the precedent of madvise's MADV_HUGEPAGE for THPs).

I don't like the interface. THP supposed to be transparent, not yet another
hugetlbs.

/sys/kernel/mm/transparent_hugepage/shmem_enabled "always" or "force"
already made this possible: but that is much too blunt an instrument,
affecting all the very different kinds of files on the internal shmem
mount, and was intended just for ease of testing hugepage loads.

I wounder if your tried "always" in production? What breaks? Maybe we can
make it work with a heuristic? This would speed up adoption.

If a tunable needed, I would rather go with fadvise(). It would operate on
a couple of bits per struct file and they get translated into VM_HUGEPAGE
and VM_NOHUGEPAGE on mmap().

Later if needed fadvise() implementation may be extended to track
requested ranges. But initially it can be simple.

-- 
 Kirill A. Shutemov