Re: [PATCH v4 bpf-next 2/4] bpf: add mmap() support for BPF_MAP_TYPE_ARRAY

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Nov 14, 2019 at 08:02:23PM -0800, 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.
> 
> One important consideration regarding how memory-mapping subsystem functions.
> Memory-mapping subsystem provides few optional callbacks, among them open()
> and close().  close() is called for each memory region that is unmapped, so
> that users can decrease their reference counters and free up resources, if
> necessary. open() is *almost* symmetrical: it's called for each memory region
> that is being mapped, **except** the very first one. So bpf_map_mmap does
> initial refcnt bump, while open() will do any extra ones after that. Thus
> number of close() calls is equal to number of open() calls plus one more.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Rik van Riel <riel@surriel.com>
> Acked-by: Song Liu <songliubraving@fb.com>
> Acked-by: John Fastabend <john.fastabend@gmail.com>
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>