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

[Alexei Starovoitov <ast@fb.com>]

On 11/15/19 3:31 PM, Daniel Borkmann wrote:
> On 11/15/19 5:02 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.
>>
>> 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>
> 
> [...]
>> +/* called for any extra memory-mapped regions (except initial) */
>> +static void bpf_map_mmap_open(struct vm_area_struct *vma)
>> +{
>> +    struct bpf_map *map = vma->vm_file->private_data;
>> +
>> +    bpf_map_inc(map);
> 
> This would also need to inc uref counter since it's technically a reference
> of this map into user space as otherwise if map->ops->map_release_uref 
> would
> be used for maps supporting mmap, then the callback would trigger even 
> if user
> space still has a reference to it.

I thought we use uref only for array that can hold FDs ?
That's why I suggested Andrii earlier to drop uref++.