Re: [PATCH 3/6] shm: add memfd_create() syscall

From: Pavel Emelyanov
Date: Thu Mar 20 2014 - 05:03:03 EST

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.

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