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Message-ID: <CAEf4BzbsJSEgnW14F7Xt+E911NC_ZqEUeLg0pxrUbaoj1Zzkyg@mail.gmail.com>
Date: Thu, 14 Nov 2019 21:43:29 -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 9:08 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Nov 14, 2019 at 09:05:26PM -0800, Andrii Nakryiko wrote:
> > 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.
>
> How mmap is going to work for hash map ? and for prog_array?
>
It probably won't. But one day we might have hash map using open
adressing, which will be more prone to memory-mapping. Or, say, some
sort of non-per-CPU ring buffer would be a good candidate as well. It
doesn't seem like a good idea to restrict mmap()-ability to just array
for no good reason.
But speaking about freeze_mutex specifically. It's to coordinate
writable memory-mapping and frozen flag. Even if we make
memory-mapping bpf_array specific, map->frozen is generic flag and
handled in syscall.c generically, so I just can't protect it only from
bpf_array side.
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