linux-kernel - Re: [PATCH 3/6] shm: add memfd

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Message-ID: <5346EBDF.2020801@mit.edu>
Date:	Thu, 10 Apr 2014 12:07:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Konstantin Khlebnikov <koct9i@...il.com>,
	David Herrmann <dh.herrmann@...il.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>,
	Karol Lewandowski <k.lewandowsk@...sung.com>,
	Kay Sievers <kay@...y.org>, Daniel Mack <zonque@...il.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kristian Høgsberg 
	<krh@...planet.net>, john.stultz@...aro.org,
	Greg Kroah-Hartman <greg@...ah.com>, Tejun Heo <tj@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ryan Lortie <desrt@...rt.ca>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 3/6] shm: add memfd_create() syscall

On 04/02/2014 06:38 AM, Konstantin Khlebnikov wrote:
> On Wed, Mar 19, 2014 at 11:06 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 explicitly allows 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() does not create a front-FD, but instead returns the raw
>> shmem file, so calls like ftruncate() can be used. Also calls like fstat()
>> will return proper information and mark the file as regular file. Sealing
>> is explicitly supported on memfds.
>>
>> Compared to O_TMPFILE, it does not require a tmpfs mount-point and is not
>> subject to quotas and alike.
> 
> Instead of adding new syscall we can extend existing openat() a little
> bit more:
> 
> openat(AT_FDSHM, "name", O_TMPFILE | O_RDWR, 0666)

Please don't.  O_TMPFILE is a messy enough API, and the last thing we
need to do is to extend it.  If we want a fancy API for creating new
inodes with no corresponding dentry, let's create one.

Otherwise, let's just stick with a special-purpose API for these shm files.

--Andy
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