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

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Date:   Thu, 14 Nov 2019 21:05:26 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        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 Thu, Nov 14, 2019 at 8:45 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> 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@...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>
> > ---
> >  include/linux/bpf.h            | 11 ++--
> >  include/linux/vmalloc.h        |  1 +
> >  include/uapi/linux/bpf.h       |  3 ++
> >  kernel/bpf/arraymap.c          | 59 +++++++++++++++++---
> >  kernel/bpf/syscall.c           | 99 ++++++++++++++++++++++++++++++++--
> >  mm/vmalloc.c                   | 20 +++++++
> >  tools/include/uapi/linux/bpf.h |  3 ++
> >  7 files changed, 184 insertions(+), 12 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 6fbe599fb977..8021fce98868 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -12,6 +12,7 @@
> >  #include <linux/err.h>
> >  #include <linux/rbtree_latch.h>
> >  #include <linux/numa.h>
> > +#include <linux/mm_types.h>
> >  #include <linux/wait.h>
> >  #include <linux/u64_stats_sync.h>
> >
> > @@ -66,6 +67,7 @@ struct bpf_map_ops {
> >                                    u64 *imm, u32 off);
> >       int (*map_direct_value_meta)(const struct bpf_map *map,
> >                                    u64 imm, u32 *off);
> > +     int (*map_mmap)(struct bpf_map *map, struct vm_area_struct *vma);
> >  };
> >
> >  struct bpf_map_memory {
> > @@ -94,9 +96,10 @@ struct bpf_map {
> >       u32 btf_value_type_id;
> >       struct btf *btf;
> >       struct bpf_map_memory memory;
> > +     char name[BPF_OBJ_NAME_LEN];
> >       bool unpriv_array;
> > -     bool frozen; /* write-once */
> > -     /* 48 bytes hole */
> > +     bool frozen; /* write-once; write-protected by freeze_mutex */
> > +     /* 22 bytes hole */
> >
> >       /* The 3rd and 4th cacheline with misc members to avoid false sharing
> >        * particularly with refcounting.
> > @@ -104,7 +107,8 @@ struct bpf_map {
> >       atomic64_t refcnt ____cacheline_aligned;
> >       atomic64_t usercnt;
> >       struct work_struct work;
> > -     char name[BPF_OBJ_NAME_LEN];
> > +     struct mutex freeze_mutex;
> > +     u64 writecnt; /* writable mmap cnt; protected by freeze_mutex */
> >  };
>
> Can the mutex be moved into bpf_array instead of being in bpf_map that is
> shared across all map types?

No, freezing logic is common to all maps. Same for writecnt and
mmap()-ing overall.

> If so then can you reuse the mutex that Daniel is adding in his patch 6/8
> of tail_call series? Not sure what would the right name for such mutex.
> It will be used for your map_freeze logic and for Daniel's text_poke.
>