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

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Date:   Fri, 15 Nov 2019 23:47:06 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 11/15/19 3:44 PM, Daniel Borkmann wrote:
> 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? :/

What do you mean 'missed to add' ?
This is mmap path. Anything that needs releasing (like FDs for 
prog_array or progs for sockmap) cannot be mmap-able.