linux-kernel - Re: [PATCH v3 3/7] shm: add memfd

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Date:	Fri, 13 Jun 2014 14:41:24 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>, Ryan Lortie <desrt@...rt.ca>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	John Stultz <john.stultz@...aro.org>,
	Lennart Poettering <lennart@...ttering.net>,
	Daniel Mack <zonque@...il.com>, Kay Sievers <kay@...y.org>,
	Hugh Dickins <hughd@...gle.com>,
	Tony Battersby <tonyb@...ernetics.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v3 3/7] shm: add memfd_create() syscall

Hi

On Fri, Jun 13, 2014 at 2:27 PM, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> Hi David,
>
> On Fri, Jun 13, 2014 at 12:36 PM, David Herrmann <dh.herrmann@...il.com> wrote:
>> memfd_create() is similar to mmap(MAP_ANON), but returns a file-descriptor
>> that you can pass to mmap(). It can support sealing and avoids any
>> connection to user-visible mount-points. Thus, it's not subject to quotas
>> on mounted file-systems, but can be used like malloc()'ed memory, but
>> with a file-descriptor to it.
>>
>> memfd_create() returns the raw shmem file, so calls like ftruncate() can
>> be used to modify the underlying inode. Also calls like fstat()
>> will return proper information and mark the file as regular file. If you
>> want sealing, you can specify MFD_ALLOW_SEALING. Otherwise, sealing is not
>> supported (like on all other regular files).
>>
>> Compared to O_TMPFILE, it does not require a tmpfs mount-point and is not
>> subject to quotas and alike. It is still properly accounted to memcg
>> limits, though.
>
> Where do I find / is there detailed documentation (ideally, a man
> page) for this new system call?

I did write a man-page proposal for memfd_create() and a patch for
fcntl() for v1, however, the API changed several times so I didn't
keep them up to date (the man-page patches are on LKML). However, I
wrote a short introduction to memfd+sealing v3, that I recommend
reading first:
  http://dvdhrm.wordpress.com/2014/06/10/memfd_create2/

This explains the idea behind the new API and describes almost all
aspects of it. It's up-to-date to v3 and I will use it to write the
final man-pages once Hugh and Andrew ACKed the patches. Let me know if
anything is unclear.

Thanks for looking at it!
David
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