On Fri, Jul 29, 2016 at 01:05:48PM -0500, Eric W. Biederman wrote: > "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> writes: > > > Hi Eric, > > > > On 07/28/2016 02:56 PM, Eric W. Biederman wrote: > >> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> 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. > 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. > > >> 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. > > >>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. I think we can avoid comparing st_dev if we will compare inode numbers for parent user namespaces. Namespaces looks like a tree where user-namespaces are directories and other namespaces are files. A namespace can be described by a path in this imaginary file system, which looks like /userns1/userns2/XXXns. In this case we need to guarantee uniq names inside each directories and that they will be not changed over migration. > > 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. > > Eric > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
