linux-kernel - Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

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: <20140314130021.GA12589@redhat.com>
Date:	Fri, 14 Mar 2014 09:00:21 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Hannes Reinecke <hare@...e.de>, Jeff Moyer <jmoyer@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dm-devel@...hat.com
Subject: Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

On Fri, Mar 14 2014 at  5:25am -0400,
Christoph Hellwig <hch@...radead.org> wrote:

> On Thu, Mar 13, 2014 at 12:13:47PM -0400, Mike Snitzer wrote:
> > Pretty ironic that in the same email that you ask someone to "Let's make
> > this a little less personal." you start by asserting upstream
> > dm-multipath sees very little testing -- and use your commit that
> > recently broke dm-multipath as the basis.  Anyway, please exapnd on what
> > you feel is broken with upstream dm-multipath.
> 
> Getting a little upset, eh?  I didn't say it's broken, I said it gets
> very little testing.  The regression from me was found like so many
> before only after it was backported o some enterprise kernel.

Even _really_ basic dm-multipath testing would've uncovered this bug.
 
> I think the problem here is two-fold:
>  a) the hardware you use with dm-multipath isn't widely available.
>  b) it uses a very special code path in the block layer no one else uses
> 
> a) might be fixable by having some RDAC or similar emulation in qemu if
> someone wants to spend the effort.

The regression from the commit in question was easily reproduced/tested
using scsi_debug.  Just start the multipathd service and any scsi_debug
device in the system will get multipath'd.

> b) is a bit harder, but we should think hard about it when rewriting the
> multipath code to support blk-mq.  Talking about which I think trying to
> use dm-multipath on any blk-mq device will go horribly crash and boom at
> the moment.

If/when blk-mq/scsi-mq is used as the primary mechanism for multipathing
it must (initially anyway) but implemented in terms of a dm-multipath
fork (call it "dm-multiqueue"?).  We cannot take 6+ months of breaking
and then fixing dm-multipath.  When dm-multiqueue is more proven we can
look at the best way forward.
--
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/