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

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Date:	Thu, 20 Mar 2014 13:01:47 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>,
	David Herrmann <dh.herrmann@...il.com>
CC:	<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@...ts.freedesktop.org>, <linux-fsdevel@...r.kernel.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 03/20/2014 12:47 PM, Cyrill Gorcunov wrote:
> On Wed, Mar 19, 2014 at 08:06:48PM +0100, David Herrmann 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.
> 
> If I'm not mistaken in something obvious, this looks similar to /proc/pid/map_files
> feature, Pavel?

Thanks, Cyrill.

It is, but the map_files will work "in the opposite direction" :) In the memfd
case one first gets an FD, then mmap()s it; in the /proc/pis/map_files case one
should first mmap() a region, then open it via /proc/self/map_files.

But I don't know whether this matters.

Thanks,
Pavel
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