linux-kernel - Re: RFC: named anonymous vmas

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Date:	Sat, 22 Jun 2013 10:30:30 -0700
From:	Colin Cross <ccross@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: RFC: named anonymous vmas

On Sat, Jun 22, 2013 at 3:31 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Fri, Jun 21, 2013 at 04:42:41PM -0700, Colin Cross wrote:
>> ranges, which John Stultz has been implementing.  The second is
>> anonymous shareable memory without having a world-writable tmpfs that
>> untrusted apps could fill with files.
>
> I still haven't seen any explanation of what ashmem buys over a shared
> mmap of /dev/zero in that respect, btw.

I believe the difference is that ashmem ties the memory to an fd, so
it can be passed to another process and mmaped to get to the same
memory, but /dev/zero does not.  Passing a /dev/zero fd and mmaping it
would result in a brand new region of zeroed memory.  Opening a tmpfs
file would allow sharing memory by passing the fd, but we don't want a
world-writable tmpfs.
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