On Fri, Feb 9, 2024 at 11:17 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Fri, 9 Feb 2024 at 05:07, Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> >
> > mmap() bpf_arena right after creation, since the kernel needs to
> > remember the address returned from mmap. This is user_vm_start.
> > LLVM will generate bpf_arena_cast_user() instructions where
> > necessary and JIT will add upper 32-bit of user_vm_start
> > to such pointers.
> >
> > Fix up bpf_map_mmap_sz() to compute mmap size as
> > map->value_size * map->max_entries for arrays and
> > PAGE_SIZE * map->max_entries for arena.
> >
> > Don't set BTF at arena creation time, since it doesn't support it.
> >
> > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> > ---
> > tools/lib/bpf/libbpf.c | 43 ++++++++++++++++++++++++++++++-----
> > tools/lib/bpf/libbpf_probes.c | 7 ++++++
> > 2 files changed, 44 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 01f407591a92..4880d623098d 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -185,6 +185,7 @@ static const char * const map_type_name[] = {
> > [BPF_MAP_TYPE_BLOOM_FILTER] = "bloom_filter",
> > [BPF_MAP_TYPE_USER_RINGBUF] = "user_ringbuf",
> > [BPF_MAP_TYPE_CGRP_STORAGE] = "cgrp_storage",
> > + [BPF_MAP_TYPE_ARENA] = "arena",
> > };
> >
> > static const char * const prog_type_name[] = {
> > @@ -1577,7 +1578,7 @@ static struct bpf_map *bpf_object__add_map(struct bpf_object *obj)
> > return map;
> > }
> >
> > -static size_t bpf_map_mmap_sz(unsigned int value_sz, unsigned int max_entries)
> > +static size_t __bpf_map_mmap_sz(unsigned int value_sz, unsigned int max_entries)
> > {
> > const long page_sz = sysconf(_SC_PAGE_SIZE);
> > size_t map_sz;
> > @@ -1587,6 +1588,20 @@ static size_t bpf_map_mmap_sz(unsigned int value_sz, unsigned int max_entries)
> > return map_sz;
> > }
> >
> > +static size_t bpf_map_mmap_sz(const struct bpf_map *map)
> > +{
> > + const long page_sz = sysconf(_SC_PAGE_SIZE);
> > +
> > + switch (map->def.type) {
> > + case BPF_MAP_TYPE_ARRAY:
> > + return __bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
> > + case BPF_MAP_TYPE_ARENA:
> > + return page_sz * map->def.max_entries;
> > + default:
> > + return 0; /* not supported */
> > + }
> > +}
> > +
> > static int bpf_map_mmap_resize(struct bpf_map *map, size_t old_sz, size_t new_sz)
> > {
> > void *mmaped;
> > @@ -1740,7 +1755,7 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
> > pr_debug("map '%s' (global data): at sec_idx %d, offset %zu, flags %x.\n",
> > map->name, map->sec_idx, map->sec_offset, def->map_flags);
> >
> > - mmap_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
> > + mmap_sz = bpf_map_mmap_sz(map);
> > map->mmaped = mmap(NULL, mmap_sz, PROT_READ | PROT_WRITE,
> > MAP_SHARED | MAP_ANONYMOUS, -1, 0);
> > if (map->mmaped == MAP_FAILED) {
> > @@ -4852,6 +4867,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
> > case BPF_MAP_TYPE_SOCKHASH:
> > case BPF_MAP_TYPE_QUEUE:
> > case BPF_MAP_TYPE_STACK:
> > + case BPF_MAP_TYPE_ARENA:
> > create_attr.btf_fd = 0;
> > create_attr.btf_key_type_id = 0;
> > create_attr.btf_value_type_id = 0;
> > @@ -4908,6 +4924,21 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
> > if (map->fd == map_fd)
> > return 0;
> >
> > + if (def->type == BPF_MAP_TYPE_ARENA) {
> > + map->mmaped = mmap((void *)map->map_extra, bpf_map_mmap_sz(map),
> > + PROT_READ | PROT_WRITE,
> > + map->map_extra ? MAP_SHARED | MAP_FIXED : MAP_SHARED,
> > + map_fd, 0);
> > + if (map->mmaped == MAP_FAILED) {
> > + err = -errno;
> > + map->mmaped = NULL;
> > + close(map_fd);
> > + pr_warn("map '%s': failed to mmap bpf_arena: %d\n",
> > + bpf_map__name(map), err);
> > + return err;
> > + }
> > + }
> > +
>
> Would it be possible to introduce a public API accessor for getting
> the value of map->mmaped?
That would be bpf_map__initial_value(), no?
> Otherwise one would have to parse through /proc/self/maps in case
> map_extra is 0.
>
> The use case is to be able to use the arena as a backing store for
> userspace malloc arenas, so that
> we can pass through malloc/mallocx calls (or class specific operator
> new) directly to malloc arena using the BPF arena.
> In such a case a lot of the burden of converting existing data
> structures or code can be avoided by making much of the process
> transparent.
> Userspace malloced objects can also be easily shared to BPF progs as a
> pool through bpf_ma style per-CPU allocator.
>
> > [...]
