Messages in this thread | | | Date | Fri, 11 Aug 2023 11:17:43 -0700 | Subject | Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links | From | Kui-Feng Lee <> |
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On 8/11/23 10:35, Martin KaFai Lau wrote: > On 8/10/23 4:15 PM, Stanislav Fomichev wrote: >> On 08/10, David Vernet wrote: >>> On Thu, Aug 10, 2023 at 03:46:18PM -0700, Stanislav Fomichev wrote: >>>> On 08/10, David Vernet wrote: >>>>> Currently, if a struct_ops map is loaded with BPF_F_LINK, it must also >>>>> define the .validate() and .update() callbacks in its corresponding >>>>> struct bpf_struct_ops in the kernel. Enabling struct_ops link is >>>>> useful >>>>> in its own right to ensure that the map is unloaded if an application >>>>> crashes. For example, with sched_ext, we want to automatically unload >>>>> the host-wide scheduler if the application crashes. We would likely >>>>> never support updating elements of a sched_ext struct_ops map, so we'd >>>>> have to implement these callbacks showing that they _can't_ support >>>>> element updates just to benefit from the basic lifetime management of >>>>> struct_ops links. >>>>> >>>>> Let's enable struct_ops maps to work with BPF_F_LINK even if they >>>>> haven't defined these callbacks, by assuming that a struct_ops map >>>>> element cannot be updated by default. >>>> >>>> Any reason this is not part of sched_ext series? As you mention, >>>> we don't seem to have such users in the three? >>> >>> Hi Stanislav, >>> >>> The sched_ext series [0] implements these callbacks. See >>> bpf_scx_update() and bpf_scx_validate(). >>> >>> [0]: https://lore.kernel.org/all/20230711011412.100319-13-tj@kernel.org/ >>> >>> We could add this into that series and remove those callbacks, but this >>> patch is fixing a UX / API issue with struct_ops links that's not really >>> relevant to sched_ext. I don't think there's any reason to couple >>> updating struct_ops map elements with allowing the kernel to manage the >>> lifetime of struct_ops maps -- just because we only have 1 (non-test) > > Agree the link-update does not necessarily couple with link-creation, so > removing 'link' update function enforcement is ok. The intention was to > avoid the struct_ops link inconsistent experience (one struct_ops link > support update and another struct_ops link does not) because consistency > was one of the reason for the true kernel backed link support that > Kui-Feng did. tcp-cc is the only one for now in struct_ops and it can > support update, so the enforcement is here. I can see Stan's point that > removing it now looks immature before a struct_ops landed in the kernel > showing it does not make sense or very hard to support 'link' update. > However, the scx patch set has shown this point, so I think it is good > enough. > > For 'validate', it is not related a 'link' update. It is for the > struct_ops 'map' update. If the loaded struct_ops map is invalid, it > will end up having a useless struct_ops map and no link can be created > from it. I can see some struct_ops subsystem check all the 'ops' > function for NULL before calling (like the FUSE RFC). I can also see > some future struct_ops will prefer not to check NULL at all and prefer > to assume a subset of the ops is always valid. Does having a 'validate' > enforcement is blocking the scx patchset in some way? If not, I would > like to keep this for now. Once it is removed, there is no turning back.
I am not saying which one is right or wrong, but the followings are some of my concerns. Just FYI!
The 'validate' change more likes a default implementation that always return 0. It is up to struct_ops types to decide how to validate values. If they decide to always success, this change will save them a bit of time. In opposite, allowing empty update may make difficulties to the developers of new struct_ops types. New developers may spend a lot of time in the code base to figure out that they should implement an update function to make it work. A better document may help. However, checking these function pointers at the first moment is even better.
> >>> struct_ops implementation in-tree doesn't mean we shouldn't improve APIs >>> where it makes sense. >>> >>> Thanks, >>> David >> >> Ack. I guess up to you and Martin. Just trying to understand whether I'm >> missing something or the patch does indeed fix some use-case :-) > >
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