linux-kernel - unshare(CLONE_VM) Re: [PATCH] unshare: Use rcu_assign

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Message-ID: <20160314131500.21b9f6c5@jules-lenovo3>
Date:	Mon, 14 Mar 2016 13:15:00 +0000
From:	Julian Smith <jsmith@...o-software.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Janak Desai <janak@...ibm.com>, Andrew Morton <akpm@...l.org>,
	ebiederm@...ssion.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: unshare(CLONE_VM) Re: [PATCH] unshare: Use rcu_assign_pointer when
 setting sighand

On Sat, 18 Mar 2006 20:24:51 +0300
Oleg Nesterov <oleg@...sign.ru> wrote:

[...]

> However, I stronly beleive unshare(CLONE_VM) is buggy.
> 
> sys_unshare:
> 
> 
> 		if (new_mm) {
> 			...
> 			new_mm = mm;
> 		}
> 
> 	...
> 
> 	bad_unshare_cleanup_vm:
> 		if (new_mm)
> 			mmput(new_mm);
> 
> 
> mmput() ignores mm->core_waiters.

Apologies for re-opening a ten-year-old thread.

I'm looking into whether it would be possible to extend the unshare
syscall to support the CLONE_VM flag with multi-threaded processes,
because this would allow us at Undo to record multi-threaded user
processes much more efficiently than at present.

We currently have to serialise threads and so suffer an N-times
slowdown when recording a process with N cpu-bound threads. But if we
could get per-thread memory permissions with unshare(CLONE_VM), we'd be
able record a multi-threaded process with almost no per-thread
slowdown.

When the unshare syscall was introduced, it seems that the
mm->core_waiters issue was the only thing that prevented CLONE_VM being
supported. Is that right, or were there other problems too?

Many thanks for any information about this.

- Julian

-- 
http://undo-software.com