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From: Hugh Dickins <hughd@google.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Shakeel Butt <shakeelb@google.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Yang Shi <shy828301@gmail.com>, Miaohe Lin <linmiaohe@huawei.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Michal Hocko <mhocko@suse.com>, Rik van Riel <riel@surriel.com>,
	Christoph Hellwig <hch@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexey Gladkov <legion@kernel.org>,
	Chris Wilson <chris@chris-wilson.co.uk>,
	Matthew Auld <matthew.auld@intel.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 07/16] memfd: memfd_create(name, MFD_HUGEPAGE) for shmem huge pages
Date: Thu, 5 Aug 2021 20:33:21 -0700 (PDT)	[thread overview]
Message-ID: <7852f33a-bfe8-cbf6-65c8-30f7c06d5e@google.com> (raw)
In-Reply-To: <20210804140341.m3ptxesrxwivqjmk@box.shutemov.name>

On Wed, 4 Aug 2021, Kirill A. Shutemov wrote:
> 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.

THP is transparent in the sense that it builds hugepages from the
normal page pool, when it can (or not when it cannot), rather than
promising hugepages from a separate pre-reserved hugetlbfs pool.

Not transparent in the sense that it cannot be limited or guided.

> 
> > /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.

We have not tried /sys/kernel/mm/transparent_hugepage/shmem_enabled
"always" in production.  Is that an experiment I want to recommend for
production?  No, I don't think so!  Why should we?

I am not looking to "speed up adoption" of huge tmpfs everywhere:
let those who find it useful use it, there is no need for it to be
used everywhere.

We have had this disagreement before: you were aiming for tmpfs on /tmp
huge=always, I didn't see the need for that; but we have always agreed
that it should not be broken there, and the better it works the better -
you did the unused_huge_shrink stuff in particular to meet such cases.

> 
> 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.

Let me shift that to the 08/16 (fcntl) response, and here answer:

> Hm, But why is the MFD_* needed if the fcntl() can do the same.

You're right, MFD_HUGEPAGE (and MFD_MEM_LOCK) are not strictly
needed if there's an fcntl() or fadvise() which can do that too.

But MFD_HUGEPAGE is the option which was first asked for, and is
the most popular usage internally - I did the fcntl at the same time,
and it has been found useful, but MFD_HUGEPAGE was the priority
(largely because fiddling with shmem_enabled interferes with
everyone's different usages, whereas huge=always on a mount
can be deployed selectively).

And it makes good sense for memfd_create() to offer MFD_HUGEPAGE,
as it is already offering MFD_HUGETLB: when we document MFD_HUGEPAGE
next to MFD_HUGETLB in the memfd_create(2) man page, that will help
developers to make a good choice.

(You said MFD_*, so I take it that you're thinking of MFD_MEM_LOCK
too: MFD_MEM_LOCK is something I added when building this series,
when I realized that it became possible once size change permitted.
Nobody here is using it yet, I don't mind if it's dropped; but it's
natural to propose it as part of the series, and it can be justified
as offering the memlock option which MFD_HUGETLB already bundles in.)

Hugh

  reply	other threads:[~2021-08-06  3:34 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30  7:22 [PATCH 00/16] tmpfs: HUGEPAGE and MEM_LOCK fcntls and memfds Hugh Dickins
2021-07-30  7:25 ` [PATCH 01/16] huge tmpfs: fix fallocate(vanilla) advance over huge pages Hugh Dickins
2021-07-30 21:36   ` Yang Shi
2021-08-01  3:38     ` Hugh Dickins
2021-08-02 20:36       ` Yang Shi
2021-07-30  7:28 ` [PATCH 02/16] huge tmpfs: fix split_huge_page() after FALLOC_FL_KEEP_SIZE Hugh Dickins
2021-07-30 23:48   ` Yang Shi
2021-07-30  7:30 ` [PATCH 03/16] huge tmpfs: remove shrinklist addition from shmem_setattr() Hugh Dickins
2021-07-30 21:50   ` Yang Shi
2021-07-30  7:36 ` [PATCH 04/16] huge tmpfs: revert shmem's use of transhuge_vma_enabled() Hugh Dickins
2021-07-30 21:56   ` Yang Shi
2021-08-01  4:01     ` Hugh Dickins
2021-08-02 20:39       ` Yang Shi
2021-07-30  7:39 ` [PATCH 05/16] huge tmpfs: move shmem_huge_enabled() upwards Hugh Dickins
2021-07-30 21:57   ` Yang Shi
2021-07-30  7:42 ` [PATCH 06/16] huge tmpfs: shmem_is_huge(vma, inode, index) Hugh Dickins
2021-07-30 23:34   ` Yang Shi
2021-08-01  5:22     ` Hugh Dickins
2021-08-01  5:37       ` Hugh Dickins
2021-08-02 21:14       ` Yang Shi
2021-08-04  8:28         ` Hugh Dickins
2021-08-04 19:01           ` Yang Shi
2021-08-06  5:21             ` Hugh Dickins
2021-08-06 17:41               ` Yang Shi
2021-08-05 23:04         ` Yang Shi
2021-08-06  5:43           ` Hugh Dickins
2021-08-06 17:57             ` Yang Shi
2021-08-12 18:19               ` Yang Shi
2021-07-30  7:45 ` [PATCH 07/16] memfd: memfd_create(name, MFD_HUGEPAGE) for shmem huge pages Hugh Dickins
2021-08-04 14:03   ` Kirill A. Shutemov
2021-08-06  3:33     ` Hugh Dickins [this message]
2021-07-30  7:48 ` [PATCH 08/16] huge tmpfs: fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE) Hugh Dickins
2021-08-04 14:08   ` Kirill A. Shutemov
2021-08-06  4:34     ` Hugh Dickins
2021-07-30  7:51 ` [PATCH 09/16] huge tmpfs: decide stat.st_blksize by shmem_is_huge() Hugh Dickins
2021-07-30 23:40   ` Yang Shi
2021-07-30  7:55 ` [PATCH 10/16] tmpfs: fcntl(fd, F_MEM_LOCK) to memlock a tmpfs file Hugh Dickins
2021-08-03  1:38   ` Matthew Wilcox
2021-08-04  9:15     ` Hugh Dickins
2021-07-30  7:57 ` [PATCH 11/16] tmpfs: fcntl(fd, F_MEM_LOCKED) to test if memlocked Hugh Dickins
2021-07-30  8:00 ` [PATCH 12/16] tmpfs: refuse memlock when fallocated beyond i_size Hugh Dickins
2021-07-30  8:03 ` [PATCH 13/16] mm: bool user_shm_lock(loff_t size, struct ucounts *) Hugh Dickins
2021-07-30  8:06 ` [PATCH 14/16] mm: user_shm_lock(,,getuc) and user_shm_unlock(,,putuc) Hugh Dickins
2021-07-30  8:09 ` [PATCH 15/16] tmpfs: permit changing size of memlocked file Hugh Dickins
2021-07-30  8:13 ` [PATCH 16/16] memfd: memfd_create(name, MFD_MEM_LOCK) for memlocked shmem Hugh Dickins

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