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

[Kirill A. Shutemov]

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