linux-kernel - Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk drivers

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: <505B1EB9020000780009CA6E@nat28.tlf.novell.com>
Date:	Thu, 20 Sep 2012 12:48:41 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"David Vrabel" <david.vrabel@...rix.com>,
	"Oliver Chick" <oliver.chick@...rix.com>
Cc:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk
 drivers

>>> On 20.09.12 at 13:30, Oliver Chick <oliver.chick@...rix.com> wrote:
> The memory overhead, and fallback mode points are related:
> -Firstly, it turns out that the overhead is actually 2.75MB, not 11MB
> per device. I made a mistake (pointed out by Jan) as the maximum number
> of requests that can fit into a single-page ring is 64, not 256.
> -Clearly, this still scales linearly. So the problem of memory footprint
> will occur with more VMs, or block devices.
> -Whilst 2.75MB per device is probably acceptable (?), if we start using
> multipage rings, then we might not want to have
> BLKIF_MAX_PERS_REQUESTS_PER_DEVICE==__RING_SIZE, as this will cause the
> memory overhead to increase. This is why I have implemented the
> 'fallback' mode. With a multipage ring, it seems reasonable to want the
> first $x$ grefs seen by blkback to be treated as persistent, and any
> later ones to be non-persistent. Does that seem sensible?

>From a resource usage pov, perhaps. But this will get the guest
entirely unpredictable performance. Plus I don't think 11Mb of
_virtual_ space is unacceptable overhead in a 64-bit kernel. If
you really want/need this in a 32-bit one, then perhaps some
other alternatives would be needed (and persistent grants may
not be the right approach there in the first place).

Jan

--
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/