Re: [PATCH bpf-next v8 06/34] bpf: prepare for memcg-based memory accounting for bpf maps

[Roman Gushchin <guro@fb.com>]

On Thu, Nov 26, 2020 at 01:21:41AM +0100, Daniel Borkmann wrote:
> On 11/25/20 4:00 AM, Roman Gushchin wrote:
> > In the absolute majority of cases if a process is making a kernel
> > allocation, it's memory cgroup is getting charged.
> > 
> > Bpf maps can be updated from an interrupt context and in such
> > case there is no process which can be charged. It makes the memory
> > accounting of bpf maps non-trivial.
> > 
> > Fortunately, after commit 4127c6504f25 ("mm: kmem: enable kernel
> > memcg accounting from interrupt contexts") and b87d8cefe43c
> > ("mm, memcg: rework remote charging API to support nesting")
> > it's finally possible.
> > 
> > To do it, a pointer to the memory cgroup of the process, which created
> > the map, is saved, and this cgroup can be charged for all allocations
> > made from an interrupt context. This commit introduces 2 helpers:
> > bpf_map_kmalloc_node() and bpf_map_alloc_percpu(). They can be used in
> > the bpf code for accounted memory allocations, both in the process and
> > interrupt contexts. In the interrupt context they're using the saved
> > memory cgroup, otherwise the current cgroup is getting charged.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Thanks for updating the cover letter; replying in this series instead
> on one more item that came to mind:
> 
> [...]
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index f3fe9f53f93c..4154c616788c 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -31,6 +31,8 @@
> >   #include <linux/poll.h>
> >   #include <linux/bpf-netns.h>
> >   #include <linux/rcupdate_trace.h>
> > +#include <linux/memcontrol.h>
> > +#include <linux/sched/mm.h>
> >   #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
> >   			  (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> > @@ -456,6 +458,77 @@ void bpf_map_free_id(struct bpf_map *map, bool do_idr_lock)
> >   		__release(&map_idr_lock);
> >   }
> > +#ifdef CONFIG_MEMCG_KMEM
> > +static void bpf_map_save_memcg(struct bpf_map *map)
> > +{
> > +	map->memcg = get_mem_cgroup_from_mm(current->mm);
> > +}
> > +
> > +static void bpf_map_release_memcg(struct bpf_map *map)
> > +{
> > +	mem_cgroup_put(map->memcg);
> > +}
> > +
> > +void *bpf_map_kmalloc_node(const struct bpf_map *map, size_t size, gfp_t flags,
> > +			   int node)
> > +{
> > +	struct mem_cgroup *old_memcg;
> > +	bool in_interrupt;
> > +	void *ptr;
> > +
> > +	/*
> > +	 * If the memory allocation is performed from an interrupt context,
> > +	 * the memory cgroup to charge can't be determined from the context
> > +	 * of the current task. Instead, we charge the memory cgroup, which
> > +	 * contained the process created the map.
> > +	 */
> > +	in_interrupt = in_interrupt();
> > +	if (in_interrupt)
> > +		old_memcg = set_active_memcg(map->memcg);
> > +
> > +	ptr = kmalloc_node(size, flags, node);
> > +
> > +	if (in_interrupt)
> > +		set_active_memcg(old_memcg);
> > +
> > +	return ptr;
> > +}
> > +
> > +void __percpu *bpf_map_alloc_percpu(const struct bpf_map *map, size_t size,
> > +				    size_t align, gfp_t gfp)
> > +{
> > +	struct mem_cgroup *old_memcg;
> > +	bool in_interrupt;
> > +	void *ptr;
> > +
> > +	/*
> > +	 * If the memory allocation is performed from an interrupt context,
> > +	 * the memory cgroup to charge can't be determined from the context
> > +	 * of the current task. Instead, we charge the memory cgroup, which
> > +	 * contained the process created the map.
> > +	 */
> > +	in_interrupt = in_interrupt();
> > +	if (in_interrupt)
> > +		old_memcg = set_active_memcg(map->memcg);
> > +
> > +	ptr = __alloc_percpu_gfp(size, align, gfp);
> > +
> > +	if (in_interrupt)
> > +		set_active_memcg(old_memcg);
> 
> For this and above bpf_map_kmalloc_node() one, wouldn't it make more sense to
> perform the temporary memcg unconditionally?
> 
> 	old_memcg = set_active_memcg(map->memcg);
> 	ptr = kmalloc_node(size, flags, node);
> 	set_active_memcg(old_memcg);
> 
> I think the semantics are otherwise a bit weird and the charging unpredictable;
> this way it would /always/ be accounted against the prog in the memcg that
> originally created the map.
> 
> E.g. maps could be shared between progs attached to, say, XDP/tc where in_interrupt()
> holds true with progs attached to skb-cgroup/egress where we're still in process
> context. So some part of the memory is charged against the original map's memcg and
> some other part against the current process' memcg which seems odd, no? Or, for example,
> if we start to run a tracing BPF prog which updates state in a BPF map ... that tracing
> prog now interferes with processes in other memcgs which may not be intentional & could
> lead to potential failures there as opposed when the tracing prog is not run. My concern
> is that the semantics are not quite clear and behavior unpredictable compared to always
> charging against map->memcg.
> 
> Similarly, what if an orchestration prog creates dedicated memcg(s) for maps with
> individual limits ... the assumed behavior (imho) would be that whatever memory is
> accounted on the map it can be accurately retrieved from there & similarly limits
> enforced, no? It seems that would not be the case currently.
> 
> Thoughts?

I did consider this option. There are pros and cons. In general we tend to charge the cgroup
which actually allocates the memory, and I decided to stick with this rule. I agree, it's fairly
easy to come with arguments why always charging the map creator is better. The opposite is
also true: it's not clear why bpf is different here. So I'm fine with both options, if there
is a wide consensus, I'm happy to switch to the other option. In general, I believe that
the current scheme is more flexible: if someone want to pay in advance, they are free to preallocate
the map. Otherwise it's up to whoever wants to populate it.

Thanks!