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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 08/16] huge tmpfs: fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE)
Date: Thu, 5 Aug 2021 21:34:20 -0700 (PDT)	[thread overview]
Message-ID: <caf82af1-1412-c8d7-e622-4f391689f8fe@google.com> (raw)
In-Reply-To: <20210804140805.vpuerwaiqtcvc5or@box.shutemov.name>

On Wed, 4 Aug 2021, Kirill A. Shutemov wrote:
> On Fri, Jul 30, 2021 at 12:48:33AM -0700, Hugh Dickins wrote:
> > Add support for fcntl(fd, F_HUGEPAGE) and fcntl(fd, F_NOHUGEPAGE), to
> > select hugeness per file: useful to override the default hugeness of the
> > shmem mount, when occasionally needing to store a hugepage file in a
> > smallpage mount or vice versa.
> 
> Hm. But why is the new MFD_* needed if the fcntl() can do the same.

That I've just addressed in the MFD_HUGEPAGE 07/16 thread.

> 
> > These fcntls just specify whether or not to try for huge pages when
> > allocating to the object later: F_HUGEPAGE does not touch small pages
> > already allocated (though khugepaged may do so when the file is mapped
> > afterwards), F_NOHUGEPAGE does not split huge pages already allocated.
> > 
> > Why fcntl?  Because it's already in use (for sealing) on memfds; and I'm
> > anxious to keep this simple, just applying it to whole files: fallocate,
> > madvise and posix_fadvise each involve a range, which would need a new
> > kind of tree attached to the inode for proper support.
> 
> Most of fadvise() operations ignore the range. I like fadvise() because
> it's less prescriptive: kernel is free to ignore it.

As to ignoring the range, yes, I see now that some do; and I'm relieved
to see "Len == 0 means as much as possible", that's great, I was afraid
of compat bugs over 0xffy numbers for the len.  And we would want, not
to ignore the range, but insist on offset 0, len 0 for now, if there's
any intention (not mine) of extending it to ranges in the future.

As to ignoring the prescription, that's just a matter of how we describe
it in the manpage, no matter whether it's fadvise() or fcntl().

And in the 07/16 thread you also said:

> 
> 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().

Not so sure about that detail: the point here is to decide what kind
of allocations to try for, before the file is mmap()ed; and it is the
file (the underlying object) that I want to condition here, rather than
the struct file of who has it open at the time, or their mmap()s.

But adding the flags into the vm_flags on mmap(): that's an interesting
idea, I haven't played with that at all.  Offhand, I don't think it will
give different allocation results from what I'm already doing, but might
affect what is shown by default in /proc/<pid>/smaps.

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

I still prefer fcntl() myself, but we can go with either: what I'd
like to hear is the preference of linux-fsdevel and linux-api people.

Aside from the unused offset+len, my main problem with fadvise()
is that... it doesn't exist.  It's posix_fadvise() or fadvise64() or
fadvise64_64(), and all its good advices are POSIX_MADV_whatever.

Are we comfortable now adding LINUX_MADV_HUGEPAGE, LINUX_MADV_NOHUGEPAGE?

I find myself singing 64 64 Zoo Lane.

Hugh

  reply	other threads:[~2021-08-06  4:35 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
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 [this message]
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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