Message-ID: <20180907001317.fj7f6fg6ihljompp@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 6 Sep 2018 17:13:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mauricio Vasquez B <mauricio.vasquez@...ito.it>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        joe@...d.net.nz
Subject: Re: [RFC PATCH bpf-next v2 0/4] Implement bpf queue/stack maps

On Fri, Aug 31, 2018 at 11:25:48PM +0200, Mauricio Vasquez B wrote:
> In some applications this is needed have a pool of free elements, like for
> example the list of free L4 ports in a SNAT.  None of the current maps allow
> to do it as it is not possibleto get an any element without having they key
> it is associated to.
> 
> This patchset implements two new kind of eBPF maps: queue and stack.
> Those maps provide to eBPF programs the peek, push and pop operations, and for
> userspace applications a new bpf_map_lookup_and_delete_elem() is added.
> 
> Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@...ito.it>
> 
> ---
> 
> I am sending this as an RFC because there is still an issue I am not sure how
> to solve.
> 
> The queue/stack maps have a linked list for saving the nodes, and a
> preallocation schema based on the pcpu_freelist already implemented and used
> in the htabmap.  Each time an element is pushed into the map, a node from the
> pcpu_freelist is taken and then added to the linked list.
> 
> The pop operation takes and *removes* the first node from the linked list, then
> it uses *call_rcu* to postpose freeing the node, i.e, the node is only returned
> to the pcpu_freelist when the rcu callback is executed.  This is needed because
> an element returned by the pop() operation should remain valid for the whole
> duration of the eBPF program.
> 
> The problem is that elements are not immediately returned to the free list, so
> in some cases the push operation could fail because there are not free nodes
> in the pcpu_freelist.
> 
> The following code snippet exposes that problem.
> 
> ...
> 	/* Push MAP_SIZE elements */
> 	for (i = 0; i < MAP_SIZE; i++)
> 		assert(bpf_map_update_elem(fd, NULL, &vals[i], 0) == 0);
> 
> 	/* Pop all elements */
> 	for (i = 0; i < MAP_SIZE; i++)
> 		assert(bpf_map_lookup_and_delete_elem(fd, NULL, &val) == 0 &&
> 		       val == vals[i]);
> 
>   // sleep(1) <-- If I put this sleep, everything works.
> 	/* Push MAP_SIZE elements */
> 	for (i = 0; i < MAP_SIZE; i++)
> 		assert(bpf_map_update_elem(fd, NULL, &vals[i], 0) == 0);
>            ^^^
>            This fails because there are not available elements in pcpu_freelist
> ...
> 
> I think a possible solution is to oversize the pcpu_freelist (no idea by how
> much, maybe double or, or make it 1.5 time the max elements in the map?)
> I also have concerns about it, it would waste that memory in many cases and
> this is also probably that it doesn't solve the issue because that code snippet
> is puhsing and popping elements too fast, so even if the pcpu_freelist is much
> large a certain time instant all the elements could be used.
> 
> Is this really an important issue?
> Any idea of how to solve it?

It is important issue indeed and a difficult one to solve.
We have the same issue with hash map.
If the prog is doing:
value = lookup(key);
delete(key);
// here the prog shouldn't be accessing the value anymore, since the memory
// could have been reused, but value pointer is still valid and points to
// allocated memory

bpf_map_pop_elem() is trying to do lookup_and_delete and preserve
validity of value without races.
With pcpu_freelist I don't think there is a solution.
We can have this queue/stack map without prealloc and use kmalloc/kfree
back and forth. Performance will not be as great, but for your use case,
I suspect, it will be good enough.
The key issue with kmalloc/kfree is unbounded time of rcu callbacks.
If somebody starts doing push/pop for every packet, the rcu subsystem
will struggle and nothing we can do about it.

The only way I could think of to resolve this problem is to reuse
the logic that Joe is working on for socket lookups inside the program.
Joe,
how is that going? Could you repost the latest patches?

In such case the api for stack map will look like:

elem = bpf_map_pop_elem(stack);
// access elem
bpf_map_free_elem(elem);
// here prog is not allowed to access elem and verifier will catch that

elem = bpf_map_alloc_elem(stack);
// populate elem
bpf_map_push_elem(elem);
// here prog is not allowed to access elem and verifier will catch that

Then both pre-allocated elems and kmalloc/kfree will work fine
and no unbounded rcu issues in both cases.

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