Re: [RFC] Per-user namespace process accounting

From: Serge Hallyn
Date: Tue Jun 03 2014 - 13:47:54 EST

Quoting Pavel Emelyanov (xemul@xxxxxxxxxxxxx):
> On 06/03/2014 09:26 PM, Serge Hallyn wrote:
> > Quoting Pavel Emelyanov (xemul@xxxxxxxxxxxxx):
> >> On 05/29/2014 07:32 PM, Serge Hallyn wrote:
> >>> Quoting Marian Marinov (mm@xxxxxx):
> >>>> Hash: SHA1
> >>>>
> >>>> On 05/29/2014 01:06 PM, Eric W. Biederman wrote:
> >>>>> Marian Marinov <mm@xxxxxx> writes:
> >>>>>
> >>>>>> Hello,
> >>>>>>
> >>>>>> I have the following proposition.
> >>>>>>
> >>>>>> Number of currently running processes is accounted at the root user namespace. The problem I'm facing is that
> >>>>>> multiple containers in different user namespaces share the process counters.
> >>>>>
> >>>>> That is deliberate.
> >>>>
> >>>> And I understand that very well ;)
> >>>>
> >>>>>
> >>>>>> So if containerX runs 100 with UID 99, containerY should have NPROC limit of above 100 in order to execute any
> >>>>>> processes with ist own UID 99.
> >>>>>>
> >>>>>> I know that some of you will tell me that I should not provision all of my containers with the same UID/GID maps,
> >>>>>> but this brings another problem.
> >>>>>>
> >>>>>> We are provisioning the containers from a template. The template has a lot of files 500k and more. And chowning
> >>>>>> these causes a lot of I/O and also slows down provisioning considerably.
> >>>>>>
> >>>>>> The other problem is that when we migrate one container from one host machine to another the IDs may be already
> >>>>>> in use on the new machine and we need to chown all the files again.
> >>>>>
> >>>>> You should have the same uid allocations for all machines in your fleet as much as possible. That has been true
> >>>>> ever since NFS was invented and is not new here. You can avoid the cost of chowning if you untar your files inside
> >>>>> of your user namespace. You can have different maps per machine if you are crazy enough to do that. You can even
> >>>>> have shared uids that you use to share files between containers as long as none of those files is setuid. And map
> >>>>> those shared files to some kind of nobody user in your user namespace.
> >>>>
> >>>> We are not using NFS. We are using a shared block storage that offers us snapshots. So provisioning new containers is
> >>>> extremely cheep and fast. Comparing that with untar is comparing a race car with Smart. Yes it can be done and no, I
> >>>> do not believe we should go backwards.
> >>>>
> >>>> We do not share filesystems between containers, we offer them block devices.
> >>>
> >>> Yes, this is a real nuisance for openstack style deployments.
> >>>
> >>> One nice solution to this imo would be a very thin stackable filesystem
> >>> which does uid shifting, or, better yet, a non-stackable way of shifting
> >>> uids at mount.
> >>
> >> I vote for non-stackable way too. Maybe on generic VFS level so that filesystems
> >> don't bother with it. From what I've seen, even simple stacking is quite a challenge.
> >
> > Do you have any ideas for how to go about it? It seems like we'd have
> > to have separate inodes per mapping for each file, which is why of
> > course stacking seems "natural" here.
> I was thinking about "lightweight mapping" which is simple shifting. Since
> we're trying to make this co-work with user-ns mappings, simple uid/gid shift
> should be enough. Please, correct me if I'm wrong.
> If I'm not, then it looks to be enough to have two per-sb or per-mnt values
> for uid and gid shift. Per-mnt for now looks more promising, since container's
> FS may be just a bind-mount from shared disk.

per-sb would work. per-mnt would as you say be nicer, but I don't see how it
can be done since parts of the vfs which get inodes but no mnt information
would not be able to figure out the shifts.

> > Trying to catch the uid/gid at every kernel-userspace crossing seems
> > like a design regression from the current userns approach. I suppose we
> > could continue in the kuid theme and introduce a iiud/igid for the
> > in-kernel inode uid/gid owners. Then allow a user privileged in some
> > ns to create a new mount associated with a different mapping for any
> > ids over which he is privileged.
> User-space crossing? From my point of view it would be enough if we just turn
> uid/gid read from disk (well, from whenever FS gets them) into uids, that would
> match the user-ns's ones, this sould cover the VFS layer and related syscalls
> only, which is, IIRC stat-s family and chown.
> Ouch, and the whole quota engine :\
> Thanks,
> Pavel
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