linux-kernel - Re: [PATCH 07/16] memfd: memfd_create(name, MFD

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Date:   Wed, 4 Aug 2021 17:03:41 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Gladkov <legion@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Matthew Auld <matthew.auld@...el.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-mm@...ck.org
Subject: 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