linux-kernel - Re: [PATCH] mm: Enable MAP_UNINITIALIZED for archs with mmu

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Message-ID: <4F1E013E.9060009@fb.com>
Date:	Mon, 23 Jan 2012 16:54:22 -0800
From:	Arun Sharma <asharma@...com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	Balbir Singh <bsingharora@...il.com>,
	<akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: Enable MAP_UNINITIALIZED for archs with mmu

On 1/18/12 6:42 PM, KAMEZAWA Hiroyuki wrote:
>
> Hmm, then,
> 1. a new task jumped into this cgroup can see any uncleared data...
> 2. if a memcg pointer is reused, the information will be leaked.

You're suggesting mm_match_cgroup() is good enough for accounting 
purposes, but not usable for cases where its important to get the 
equality right?

> 3. If VM_UNINITALIZED is set, the process can see any data which
>     was freed by other process which doesn't know VM_UNINITALIZED at all.
>
> 4. The process will be able to see file cache data which the it has no
>     access right if it's accessed by memcg once.
>
> 3&  4 seems too danger.

Yes - these are the risks that I'm hoping we can document, so the 
cgroups admin can avoid opting-in if not everything running in the 
cgroup is trusted.

>
> Isn't it better to have this as per-task rather than per-memcg ?
> And just allow to reuse pages the page has freed ?
>

I'm worrying that the additional complexity of maintaining a per-task 
page list would be a problem. It might slow down workloads that 
alloc/free a lot because of the added code. It'll probably touch the 
kswapd as well (for reclaiming pages from the per-task free lists under 
low mem conditions).

Did you have some implementation ideas which would not have the problems 
above?

  -Arun
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