Re: [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call

From: Alexei Starovoitov
Date: Thu Jul 25 2019 - 19:54:38 EST

On Thu, Jul 25, 2019 at 04:25:53PM -0700, Brian Vazquez wrote:
> > > > If prev_key is deleted before map_get_next_key(), we get the first key
> > > > again. This is pretty weird.
> > >
> > > Yes, I know. But note that the current scenario happens even for the
> > > old interface (imagine you are walking a map from userspace and you
> > > tried get_next_key the prev_key was removed, you will start again from
> > > the beginning without noticing it).
> > > I tried to sent a patch in the past but I was missing some context:
> > > before NULL was used to get the very first_key the interface relied in
> > > a random (non existent) key to retrieve the first_key in the map, and
> > > I was told what we still have to support that scenario.
> >
> > BPF_MAP_DUMP is slightly different, as you may return the first key
> > multiple times in the same call. Also, BPF_MAP_DUMP is new, so we
> > don't have to support legacy scenarios.
> >
> > Since BPF_MAP_DUMP keeps a list of elements. It is possible to try
> > to look up previous keys. Would something down this direction work?
> I've been thinking about it and I think first we need a way to detect
> that since key was not present we got the first_key instead:
> - One solution I had in mind was to explicitly asked for the first key
> with map_get_next_key(map, NULL, first_key) and while walking the map
> check that map_get_next_key(map, prev_key, key) doesn't return the
> same key. This could be done using memcmp.
> - Discussing with Stan, he mentioned that another option is to support
> a flag in map_get_next_key to let it know that we want an error
> instead of the first_key.
> After detecting the problem we also need to define what we want to do,
> here some options:
> a) Return the error to the caller
> b) Try with previous keys if any (which be limited to the keys that we
> have traversed so far in this dump call)
> c) continue with next entries in the map. array is easy just get the
> next valid key (starting on i+1), but hmap might be difficult since
> starting on the next bucket could potentially skip some keys that were
> concurrently added to the same bucket where key used to be, and
> starting on the same bucket could lead us to return repeated elements.
> Or maybe we could support those 3 cases via flags and let the caller
> decide which one to use?

this type of indecision is the reason why I wasn't excited about
batch dumping in the first place and gave 'soft yes' when Stan
mentioned it during lsf/mm/bpf uconf.
We probably shouldn't do it.
It feels this map_dump makes api more complex and doesn't really
give much benefit to the user other than large map dump becomes faster.
I think we gotta solve this problem differently.