Re: [PATCH 11/11] blkcg: implement per-blkg request allocation

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Apr 27, 2012 at 11:40:34AM -0400, Vivek Goyal wrote:
> On Fri, Apr 27, 2012 at 08:02:17AM -0700, Tejun Heo wrote:
> > Hello,
> > 
> > On Fri, Apr 27, 2012 at 10:54:01AM -0400, Jeff Moyer wrote:
> > > > This patch implements per-blkg request_list.  Each blkg has its own
> > > > request_list and any IO allocates its request from the matching blkg
> > > > making blkcgs completely isolated in terms of request allocation.
> > > 
> > > So, nr_requests is now actually nr_requests * # of blk cgroups.  Is that
> > > right?  Are you at all concerned about the amount of memory that can be
> > > tied up as the number of cgroups increases?
> > 
> > Yeah, I thought about it and I don't think there's a single good
> > solution here.  The other extreme would be splitting nr_requests by
> > the number of cgroups but that seems even worse - each cgroup should
> > be able to hit maximum throughput.  Given that a lot of workloads tend
> > to regulate themselves before hitting nr_requests, I think it's best
> > to leave it as-is and treat each cgroup as having separate channel for
> > now.  It's a configurable parameter after all.
> 
> So on a slow device a malicious application can easily create thousands
> of group, queue up tons of IO and create unreclaimable memory easily?
> Sounds little scary. 

Malicious application may just jack up nr_requests.

Thanks.

-- 
tejun