Date:   Thu, 25 Jul 2019 18:47:10 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Brian Vazquez <brianvv.kernel@...il.com>
Cc:     Song Liu <liu.song.a23@...il.com>,
        Brian Vazquez <brianvv@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>,
        Stanislav Fomichev <sdf@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more
 than one entry per call

On Thu, Jul 25, 2019 at 6:24 PM Brian Vazquez <brianvv.kernel@...il.com> wrote:
>
> On Thu, Jul 25, 2019 at 4:54 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > 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.
>
> Some users are working around the dumping problems with the existing
> api by creating a bpf_map_get_next_key_and_delete userspace function
> (see https://www.bouncybouncy.net/blog/bpf_map_get_next_key-pitfalls/)
> which in my opinion is actually a good idea. The only problem with
> that is that calling bpf_map_get_next_key(fd, key, next_key) and then
> bpf_map_delete_elem(fd, key) from userspace is racing with kernel code
> and it might lose some information when deleting.
> We could then do map_dump_and_delete using that idea but in the kernel
> where we could better handle the racing condition. In that scenario
> even if we retrieve the same key it will contain different info ( the
> delta between old and new value). Would that work?

you mean get_next+lookup+delete at once?
Sounds useful.
Yonghong has been thinking about batching api as well.

I think if we cannot figure out how to make a batch of two commands
get_next + lookup to work correctly then we need to identify/invent one
command and make batching more generic.
Like make one jumbo/compound/atomic command to be get_next+lookup+delete.
Define the semantics of this single compound command.
And then let batching to be a multiplier of such command.
In a sense that multiplier 1 or N should be have the same way.
No extra flags to alter the batching.
The high level description of the batch would be:
pls execute get_next,lookup,delete and repeat it N times.
or
pls execute get_next,lookup and repeat N times.
where each command action is defined to be composable.

Just a rough idea.

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