linux-kernel - Re: [PATCH 4/4] add ksm kernel shared memory driver.

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Message-ID: <49D23224.9000903@codemonkey.ws>
Date:	Tue, 31 Mar 2009 10:09:24 -0500
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	Izik Eidus <ieidus@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, avi@...hat.com,
	chrisw@...hat.com, riel@...hat.com, jeremy@...p.org,
	mtosatti@...hat.com, hugh@...itas.com, corbet@....net,
	yaniv@...hat.com, dmonakhov@...nvz.org
Subject: Re: [PATCH 4/4] add ksm kernel shared memory driver.

Andrea Arcangeli wrote:
> On Tue, Mar 31, 2009 at 09:37:17AM -0500, Anthony Liguori wrote:
>   
>> In the very least, if you insist on not using sysfs, you should have a 
>> separate character device that's used for control (like /dev/ksmctl).
>>     
>
> I'm fine to use sysfs that's not the point, if you've to add a ksmctl
> device, then sysfs is surely better. Besides ksm would normally be
> enabled at boot, tasks jailed by selinux will better not start/stop
> this thing.
>
> If people wants /sys/kernel/mm/ksm instead of the start_stop ioctl we
> surely can add it (provided there's a way to intercept write to the
> sysfs file). Problem is registering memory could also be done with
> 'echo 0 -1 >/proc/self/ksm' and be inherited by childs, it's not just
> start/stop. I mean this is more a matter of taste I'm
> afraid... Personally I'm more concerned about the registering of the
> ram API than the start/stop thing which I cannot care less about,

I don't think the registering of ram should be done via sysfs.  That 
would be a pretty bad interface IMHO.  But I do think the functionality 
that ksmctl provides along with the security issues I mentioned earlier 
really suggest that there ought to be a separate API for control vs. 
registration and that control API would make a lot of sense as a sysfs API.

If you wanted to explore alternative APIs for registration, madvise() 
seems like the obvious candidate to me.

madvise(start, size, MADV_SHARABLE) seems like a pretty obvious API to me.

So combining a sysfs interface for control and an madvise() interface 
for registration seems like a really nice interface to me.

Regards,

Anthony Liguori

>  so
> my logic is that as long as this pseudodevice exists, we should use it
> for everything. If we go away from it, then we should remove it as a
> whole.
>   

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