linux-kernel - Re: [PATCH RFC 4/4] xen-block: introduce a new request type to unmap grants

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <51DC3C66.2030606@citrix.com>
Date:	Tue, 9 Jul 2013 18:37:58 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 4/4] xen-block: introduce a new request type to unmap
 grants

On 08/07/13 21:41, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 08, 2013 at 03:03:27PM +0200, Roger Pau Monne wrote:
>> Right now blkfront has no way to unmap grant refs, if using persistent
>> grants once a grant is used blkfront cannot assure if blkback will
>> have this grant mapped or not. To solve this problem, a new request
>> type (BLKIF_OP_UNMAP) that allows requesting blkback to unmap certain
>> grants is introduced.
> 
> I don't think this is the right way of doing it. It is a new operation
> (BLKIF_OP_UNMAP) that has nothing to do with READ/WRITE. All it is
> is just some way for the frontend to say: unmap this grant if you can.
> 
> As such I would think a better mechanism would be to have a new
> grant mechanism that can say: 'I am done with this grant you can
> remove it' - that is called to the hypervisor. The hypervisor
> can then figure out whether it is free or not and lazily delete it.
> (And the guest would be notified when it is freed).

I would prefer not to involve the hypervisor in persistent grants, this
is something between the frontends and the backends. The hypervisor
already provides the basic operations (map/unmap), IMHO there's no need
to add more logic to the hypervisor itself.

I agree that it would be better to have a generic way to request a
backend to unmap certain grants, but so far this seems like the best
solution.

> 
> I would presume that this problem would also exist with netback/netfront
> if it started using persisten grants, right?

I'm not sure of that, it depends on the number of persistent grants
netfront/netback use, in the block case we need this operation because
of indirect descriptors, but netfront/netback might not suffer from this
problem if the maximum number of grants they use is relatively small.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/