Date:   Sat, 16 Nov 2019 00:44:45 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <ast@...com>, Andrii Nakryiko <andriin@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        Kernel Team <Kernel-team@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4 bpf-next 2/4] bpf: add mmap() support for
 BPF_MAP_TYPE_ARRAY

On 11/16/19 12:37 AM, Alexei Starovoitov wrote:
> 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@...xchg.org>
>>> Cc: Rik van Riel <riel@...riel.com>
>>> Acked-by: Song Liu <songliubraving@...com>
>>> Acked-by: John Fastabend <john.fastabend@...il.com>
>>> Signed-off-by: Andrii Nakryiko <andriin@...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++.

Yeah, only for fd array currently. Question is, if we ever reuse that map_release_uref
callback in future for something else, will we remember that we earlier missed to add
it here? :/

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