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

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

On Thu, Nov 7, 2019 at 10:39 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Nov 7, 2019, at 8:20 PM, Andrii Nakryiko <andriin@fb.com> 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 arrays are supported right now. Maps with spinlocks can't be
> > memory mapped either.
> >
> > Cc: Rik van Riel <riel@surriel.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
>
> Acked-by: Song Liu <songliubraving@fb.com>
>
> With one nit below.
>
>
> [...]
>
> > -     if (percpu)
> > +     data_size = 0;
> > +     if (percpu) {
> >               array_size += (u64) max_entries * sizeof(void *);
> > -     else
> > -             array_size += (u64) max_entries * elem_size;
>
> > +     } else {
> > +             if (attr->map_flags & BPF_F_MMAPABLE) {
> > +                     data_size = (u64) max_entries * elem_size;
> > +                     data_size = round_up(data_size, PAGE_SIZE);
> > +             } else {
> > +                     array_size += (u64) max_entries * elem_size;
> > +             }
> > +     }
> >
> >       /* make sure there is no u32 overflow later in round_up() */
> > -     cost = array_size;
> > +     cost = array_size + data_size;
>
>
>
> This is a little confusing. Maybe we can do
>

I don't think I can do that without even bigger code churn. In
non-mmap()-able case, array_size specifies the size of one chunk of
memory, which consists of sizeof(struct bpf_array) bytes, followed by
actual data. This is accomplished in one allocation. That's current
case for arrays.

For BPF_F_MMAPABLE case, though, we have to do 2 separate allocations,
to make sure that mmap()-able part is allocated with vmalloc() and is
page-aligned. So array_size keeps track of number of bytes allocated
for struct bpf_array plus, optionally, per-cpu or non-mmapable array
data, while data_size is explicitly for vmalloc()-ed mmap()-able chunk
of data. If not for this, I'd just keep adjusting array_size.

So the invariant for per-cpu and non-mmapable case is that data_size =
0, array_size = sizeof(struct bpf_array) + whatever amount of data we
need. For mmapable case: array_size = sizeof(struct bpf_array),
data_size = actual amount of array data.


>         data_size = (u64) max_entries * (per_cpu ? sizeof(void *) : elem_size;
>         if (attr->map_flags & BPF_F_MMAPABLE)
>                 data_size = round_up(data_size, PAGE_SIZE);
>
>         cost = array_size + data_size;
>
> So we use data_size in all cases.
>
> Maybe also rename array_size.
>
>
> >       if (percpu)
> >               cost += (u64)attr->max_entries * elem_size * num_possible_cpus();
>
> And maybe we can also include this in data_size.

see above.

>
> [...]
>