Re: What's going on with vnets and epairs w/ addresses?
- In reply to: Mark Johnston : "Re: What's going on with vnets and epairs w/ addresses?"
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Date: Thu, 22 Dec 2022 15:38:07 UTC
> > On Dec 21, 2022, at 12:12 AM, Mark Johnston <markj@freebsd.org <mailto:markj@freebsd.org>> wrote: > > On Sun, Dec 18, 2022 at 10:52:58AM -0600, Kyle Evans wrote: >> On Sat, Dec 17, 2022 at 11:22 AM Gleb Smirnoff <glebius@freebsd.org <mailto:glebius@freebsd.org>> wrote: >>> >>> Zhenlei, >>> >>> On Fri, Dec 16, 2022 at 06:30:57PM +0800, Zhenlei Huang wrote: >>> Z> I managed to repeat this issue on CURRENT/14 with this small snip: >>> Z> >>> Z> ------------------------------------------- >>> Z> #!/bin/sh >>> Z> >>> Z> # test jail name >>> Z> n="test_ref_leak" >>> Z> >>> Z> jail -c name=$n path=/ vnet persist >>> Z> # The following line trigger jail pr_ref leak >>> Z> jexec $n ifconfig lo0 inet 127.0.0.1/8 >>> Z> >>> Z> jail -R $n >>> Z> >>> Z> # wait a moment >>> Z> sleep 1 >>> Z> >>> Z> jls -j $n >>> Z> >>> Z> After DDB debugging and tracing , it seems that is triggered by a combine of [1] and [2] >>> Z> >>> Z> [1] https://reviews.freebsd.org/rGfec8a8c7cbe4384c7e61d376f3aa5be5ac895915 <https://reviews.freebsd.org/rGfec8a8c7cbe4384c7e61d376f3aa5be5ac895915> <https://reviews.freebsd.org/rGfec8a8c7cbe4384c7e61d376f3aa5be5ac895915 <https://reviews.freebsd.org/rGfec8a8c7cbe4384c7e61d376f3aa5be5ac895915>> >>> Z> [2] https://reviews.freebsd.org/rGeb93b99d698674e3b1cc7139fda98e2b175b8c5b <https://reviews.freebsd.org/rGeb93b99d698674e3b1cc7139fda98e2b175b8c5b> <https://reviews.freebsd.org/rGeb93b99d698674e3b1cc7139fda98e2b175b8c5b <https://reviews.freebsd.org/rGeb93b99d698674e3b1cc7139fda98e2b175b8c5b>> >>> Z> >>> Z> >>> Z> In [1] the per-VNET uma zone is shared with the global one. >>> Z> `pcbinfo->ipi_zone = pcbstor->ips_zone;` >>> Z> >>> Z> In [2] unref `inp->inp_cred` is deferred called in inpcb_dtor() by uma_zfree_smr() . >>> Z> >>> Z> Unfortunately inps freed by uma_zfree_smr() are cached and inpcb_dtor() is not called immediately , >>> Z> thus leaking `inp->inp_cred` ref and hence `prison->pr_ref`. >>> Z> >>> Z> And it is also not possible to free up the cache by per-VNET SYSUNINIT tcp_destroy / udp_destroy / rip_destroy. >>> >>> This is known issue and I'd prefer not to call it a problem. The "leak" of a jail >>> happens only if machine is idle wrt the networking activity. >>> >>> Getting back to the problem that started this thread - the epair(4)s not immediately >>> popping back to prison0. IMHO, the problem again lies in the design of if_vmove and >>> epair(4) in particular. The if_vmove shall not exist, instead we should do a full >>> if_attach() and if_detach(). The state of an ifnet when it undergoes if_vmove doesn't >>> carry any useful information. With Alexander melifaro@ we discussed better options >>> for creating or attaching interfaces to jails that if_vmove. Until they are ready >>> the most easy workaround to deal with annoying epair(4) come back problem is to >>> remove it manually before destroying a jail, like I did in 80fc25025ff. >>> >> >> It still behaved much better prior to eb93b99d6986, which you and Mark >> were going to work on a solution for to allow the cred "leak" to close >> up much more quickly. CC markj@, since I think it's been six months >> since the last time I inquired about it, making this a good time to do >> it again... > > I spent some time trying to see if we could fix this in UMA/SMR and > talked to Jeff about it a bit. At this point I don't think it's the > right approach, at least for now. Really we have a composability > problem where different layers are using different techniques to signal > that they're done with a particular piece of memory, and they just > aren't compatible. I originally thought that `uma_free_smr()` is somewhat like `epoch_call()` with an internal `epoch_callback_t`, but after digging into the source code it is not true. `uma_free_smr()` put the item into cache and until next allocation from the cache the destructor get a chance to run. Can SMR provide some mean just like `epoch_callback_t` , so that the destructors eventually get been invoked ? > > One thing I tried is to implement a UMA function which walks over all > SMR zones and synchronizes all cached items (so that their destructors > are called). This is really expensive, at minimum it has to bind to all > CPUs in the system so that it can flush per-CPU buckets. If > jail_deref() calls that function, the bug goes away at least in my > limited testing, but its use is really a layering violation. I've proposed a `vnet_shutdown()` stage in another mail. Maybe we can introduce a `vnet_cleanup()` and INPCB layer register to listen a `cleanup` event and the function which synchronizing cached items get been invoked. Is that still a layering violation? > > We could, say, periodically scan cached UMA/SMR items and invoke their > destructors, but for most SMR consumers this is unnecessary, and again > there's a layering problem: the inpcb layer shouldn't "know" that it has > to do that for its zones, since it's the jail layer that actually cares. > > It also seems kind of strange that dying jails still occupy a slot in > the jail namespace. I don't really understand why the existence of a > dying jail prevents creation of a new jail with the same name, but > presumably there's a good reason for it? > > Now my inclination is to try and fix this in the inpcb layer, by not > accessing the inp_cred at all in the lookup path until we hold the inpcb > lock, and then releasing the cred ref before freeing a PCB to its zone. > I think this is doable based on a few observations: > - When doing an SMR-protected lookup, we always lock the returned inpcb > before handing it to the caller. So we could in principle perform > inp_cred checking after acquiring the lock but before returning. > - If there are no jailed PCBs in a hash chain in_pcblookup_hash_locked() > always scans the whole chain. > - If we match only one PCB in a lookup, we can probably(?) return that > PCB without dereferencing the cred pointer at all. If not, then the > scan only has to keep track of a fixed number of PCBs before picking > which one to return. So it looks like we can perform a lockless scan > and keep track of matches on the stack, then lock the matched PCBs and > perform prison checks if necessary, without making the common case > more expensive. > > In fact there is a parallel thread on freebsd-jail which reports that > this inp_cred access is a source of frequent cache misses. I was > surprised to see that the scan calls prison_flag() before even checking > the PCB's local address. So if the hash chain is large then we're > potentially performing a lot of unnecessary memory accesses (though > presumably it's common for most of the PCBs to be sharing a single > cred?). In particular we can perhaps solve two problems at once. > > Any thoughts? Are there some fundamental reasons this can't work?