[CRIU] [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces

[Michael Kerrisk (man-pages)]

Hi Eric,

On 07/29/2016 08:05 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com> writes:
>
>> Hi Eric,
>>
>> On 07/28/2016 02:56 PM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages at gmail.com> writes:
>>>
>>>> On 07/26/2016 10:39 PM, Andrew Vagin wrote:
>>>>> On Tue, Jul 26, 2016 at 09:17:31PM +0200, Michael Kerrisk (man-pages) wrote:
>>>
>>>>> If we want to compare two file descriptors of the current process,
>>>>> it is one of cases for which kcmp can be used. We can call kcmp to
>>>>> compare two namespaces which are opened in other processes.
>>>>
>>>> Is there really a use case there? I assume we're talking about the
>>>> scenario where a process in one namespace opens a /proc/PID/ns/*
>>>> file descriptor and passes that FD to another process via a UNIX
>>>> domain socket. Is that correct?
>>>>
>>>> So, supposing that we want to build a map of the relationships
>>>> between namespaces using the proposed kcmp() API, and there are
>>>> say N namespaces? Does this mena we make (N * (N-1) / 2) calls
>>>> to kcmp()?
>>>
>>> Potentially.  The numbers are small enough O(N^2) isn't fatal.
>>
>> Define "small", please.
>>
>> O(N^2) makes me nervous about what other use cases lurk out
>> there that may get bitten by this.
>
> Worst case for N (One namespace per thread) is about 60k.

I'm getting an education here: where does the 60k number come from?

> A typical heavy use case may be 1000 namespaces of any type.
> So we are talking about O(N^2) that rarely happens and should be done in
> a couple of seconds.

I don't know whether that's acceptable for the migration use case,
but seems quite bad for the visualization use case.

>>> Where kcmp shines is that it allows migration to happen.  Inode numbers
>>> to change (which they very much will today), and still have things work.
>>
>>
>>> We can keep it O(Nlog(N)) by taking advantage of not just the equality
>>> but the ordering relationship.  Although Ugh.
>>
>> Yes, that sounds pretty ugly...
>
> Actually having thought about this a little more if kcmp returns an
> ordering by inode and migration preserves the relative order of
> the inodes (which should just be a creation order) it should be quite
> solvable.
>
> Switch from an order by inode number to an order by object creation
> time, and guarantee that all creations are have an order (which with
> task_list_lock we practically already have) and it should be even easier
> to create.  (A 64bit nanosecond resolution timestamp is good for 544
> years of uptime).  A 64bit number that increments each time an object is
> created should have an even better lifespan.
>
> I don't know if we can find a way to give that guarantee for other kcmp
> comparisons but it is worth a thought.

Okay. So, this is a pathway to O(Nlog(N)) at least then?

>>> One disadvantage of
>>> kcmp currently is that the way the ordering relationship is defined
>>> the order is not preserved over migration :(
>>
>> So, does kcmp() fully solve the proble(s) at hand? It sounds like
>> not, if I understand your last point correctly.
>
> There are 3 possibilities I see for migration in migration, ordered
> in order of implementation difficulty.
> 1) Have a clear signal that migration happened and a nested migration
>    needs to restart.
> 2) Use kcmp so that only the relative order needs to be preserved.
> 3) Preserve the device number and inode numbers.
>
> At a practical level I think (2) may actually in net be the simplest.
> It requires a little more care to implement and you have to opt in,
> but it should not require any rolling back of activity (merely careful
> ordering of object creation).
>
> I definititely like kcmp knowing how to compare things by inode
> (aka st_dev, st_inode) because then even if you have to restart
> the comparisons after a migration the exact details you are comparing
> are hidden and so it is easier to support and harder to get wrong.
>
> I can imagine how to preserve inode numbers by creating a new instance
> of nsfs instance and using the old inode numbers upon restore.  I don't
> currently see how we could possibly preserve st_dev over migration short of
> a device number namespace.
>
> So if we are going to continue with making device numbers be a legacy
> attribute applications should not care about we need a way to compare
> things by not looking at st_dev.  Which brings us back to kcmp.
>
> Hmm.  Hotplugging as disk and plugging it back likely will change the
> device number and give the same kind of challenge with st_dev (although
> you can't keep a file descriptor open across that kind of event).  So
> certainly a hotplug event on a device should be enough to say don't care
> about the device number.

Okay.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/