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

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Date:	Thu, 20 Mar 2014 15:50:30 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	David Herrmann <dh.herrmann@...il.com>
CC:	Cyrill Gorcunov <gorcunov@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 <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" <dri-devel@...ts.freedesktop.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <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 03:29 PM, David Herrmann wrote:
> Hi
> 
> On Thu, Mar 20, 2014 at 10:01 AM, Pavel Emelyanov <xemul@...allels.com> wrote:
>> On 03/20/2014 12:47 PM, Cyrill Gorcunov wrote:
>>> If I'm not mistaken in something obvious, this looks similar to /proc/pid/map_files
>>> feature, Pavel?
>>
>> 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.
> 
> Yes, you can replace memfd_create() so far with:
>   p = mmap(NULL, size, ..., MAP_ANON | MAP_SHARED, -1, 0);
>   sprintf(path, "/proc/self/map_files/%lx-%lx", p, p + size);
>   fd = open(path, O_RDWR);
> 
> However, map_files is only enabled with CONFIG_CHECKPOINT_RESTORE, the
> /proc/pid/map_files/ directory is root-only (at least I get EPERM if
> non-root),

Yes. But this is something we'd also like to have fixed :) Having two
parties willing the same makes it easier for the patch to get accepted.

> it doesn't provide the "name" argument which is very handy
> for debugging,

What if we make mmap's shmem_zero_setup() generate a meaningful name,
would it solve the debugging issue?

> it doesn't explicitly support sealing (it requires MAP_ANON to be backed 
> by shmem)

Can you elaborate on this? The fd generated by sys_memfd() will be
shmem-backed, so will be the file opened via map_files link for the
MAP_ANON | MAP_SHARED mapping. So what are the problems to make it
support sealing?

> and it's a very weird API for something this simple.

:)

Thanks,
Pavel
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