Re: [PATCH v3 bpf-next 1/3] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Nov 13, 2019 at 12:38 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 11/13/19 4:15 AM, Andrii Nakryiko wrote:
> > Add ability to memory-map contents of BPF array map. This is extremely useful
> > for working with BPF global data from userspace programs. It allows to avoid
> > typical bpf_map_{lookup,update}_elem operations, improving both performance
> > and usability.
> >
> > There had to be special considerations for map freezing, to avoid having
> > writable memory view into a frozen map. To solve this issue, map freezing and
> > mmap-ing is happening under mutex now:
> >    - if map is already frozen, no writable mapping is allowed;
> >    - if map has writable memory mappings active (accounted in map->writecnt),
> >      map freezing will keep failing with -EBUSY;
> >    - once number of writable memory mappings drops to zero, map freezing can be
> >      performed again.
> >
> > Only non-per-CPU plain arrays are supported right now. Maps with spinlocks
> > can't be memory mapped either.
> >
> > For BPF_F_MMAPABLE array, memory allocation has to be done through vmalloc()
> > to be mmap()'able. We also need to make sure that array data memory is
> > page-sized and page-aligned, so we over-allocate memory in such a way that
> > struct bpf_array is at the end of a single page of memory with array->value
> > being aligned with the start of the second page. On deallocation we need to
> > accomodate this memory arrangement to free vmalloc()'ed memory correctly.
> >
> > Cc: Rik van Riel <riel@surriel.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: Song Liu <songliubraving@fb.com>
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
>
> Overall set looks good to me! One comment below:
>
> [...]
> > @@ -117,7 +131,20 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
> >               return ERR_PTR(ret);
> >
> >       /* allocate all map elements and zero-initialize them */
> > -     array = bpf_map_area_alloc(array_size, numa_node);
> > +     if (attr->map_flags & BPF_F_MMAPABLE) {
> > +             void *data;
> > +
> > +             /* kmalloc'ed memory can't be mmap'ed, use explicit vmalloc */
> > +             data = vzalloc_node(array_size, numa_node);
> > +             if (!data) {
> > +                     bpf_map_charge_finish(&mem);
> > +                     return ERR_PTR(-ENOMEM);
> > +             }
> > +             array = data + round_up(sizeof(struct bpf_array), PAGE_SIZE)
> > +                     - offsetof(struct bpf_array, value);
> > +     } else {
> > +             array = bpf_map_area_alloc(array_size, numa_node);
> > +     }
>
> Can't we place/extend all this logic inside bpf_map_area_alloc() and
> bpf_map_area_free() API instead of hard-coding it here?
>
> Given this is a generic feature of which global data is just one consumer,
> my concern is that this reintroduces similar issues that mentioned API was
> trying to solve already meaning failing early instead of trying hard and
> triggering OOM if the array is large.
>
> Consolidating this into bpf_map_area_alloc()/bpf_map_area_free() would
> make sure all the rest has same semantics.

So a bunch of this (e.g, array pointer adjustment in mmapable case)
depends on specific layout of bpf_array, while bpf_map_area_alloc() is
called for multitude of different maps. What we can generalize,
though, is this enforcement of vmalloc() for mmapable case: enforce
size is multiple of PAGE_SIZE, bypass kmalloc, etc. I can do that part
easily, I refrained because it would require extra bool mmapable flag
to bpf_map_area_alloc() and (trivial) update to 13 call sites passing
false, I wasn't sure people would like code churn.

As for bpf_map_areas_free(), again, adjustment is specific to
bpf_array and its memory layout w.r.t. data placement, so I don't
think we can generalize it that much.

After talking with Johannes, I'm also adding new
vmalloc_user_node_flags() API and will specify same RETRY_MAYFAIL and
NOWARN flags, so behavior will stay the same.

Let me know if you want `bool mmapable` added to bpf_map_area_alloc().
And also if I'm missing how you wanted to generalize other parts,
please explain in more details.

>
> >       if (!array) {
> >               bpf_map_charge_finish(&mem);
> >               return ERR_PTR(-ENOMEM);
> > @@ -365,7 +392,10 @@ static void array_map_free(struct bpf_map *map)
> >       if (array->map.map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
> >               bpf_array_free_percpu(array);
> >
> > -     bpf_map_area_free(array);
> > +     if (array->map.map_flags & BPF_F_MMAPABLE)
> > +             bpf_map_area_free((void *)round_down((long)array, PAGE_SIZE));
> > +     else
> > +             bpf_map_area_free(array);
> >   }
> >
> >   static void array_map_seq_show_elem(struct bpf_map *map, void *key,
> [...]