Messages in this thread | | | Subject | Re: [PATCH 0/5 RFC] Add an interface to discover relationships between namespaces | From | "Michael Kerrisk (man-pages)" <> | Date | Sun, 31 Jul 2016 23:31:44 +0200 |
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Hi Eric,
On 07/29/2016 08:05 PM, Eric W. Biederman wrote: > "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes: > >> Hi Eric, >> >> On 07/28/2016 02:56 PM, Eric W. Biederman wrote: >>> "Michael Kerrisk (man-pages)" <mtk.manpages@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/
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