On 7/15/22 12:24 PM, Andrii Nakryiko wrote:
Make libbpf adjust RINGBUF map size (rounding it up to closest power-of-2
of page_size) more eagerly: during open phase when initializing the map
and on explicit calls to bpf_map__set_max_entries().
Such approach allows user to check actual size of BPF ringbuf even
before it's created in the kernel, but also it prevents various edge
case scenarios where BPF ringbuf size can get out of sync with what it
would be in kernel. One of them (reported in [0]) is during an attempt
to pin/reuse BPF ringbuf.
Move adjust_ringbuf_sz() helper closer to its first actual use. The
implementation of the helper is unchanged.
[0] Closes: https://github.com/libbpf/libbpf/issue/530
The link is not accessible. https://github.com/libbpf/libbpf/pull/530
is accessible.
Fixes: 0087a681fa8c ("libbpf: Automatically fix up BPF_MAP_TYPE_RINGBUF size, if necessary")
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 77 +++++++++++++++++++++++-------------------
1 file changed, 42 insertions(+), 35 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 68da1aca406c..2767d1897b4f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2320,6 +2320,37 @@ int parse_btf_map_def(const char *map_name, struct btf *btf,
return 0;
}
+static size_t adjust_ringbuf_sz(size_t sz)
+{
+ __u32 page_sz = sysconf(_SC_PAGE_SIZE);
+ __u32 mul;
+
+ /* if user forgot to set any size, make sure they see error */
+ if (sz == 0)
+ return 0;
+ /* Kernel expects BPF_MAP_TYPE_RINGBUF's max_entries to be
+ * a power-of-2 multiple of kernel's page size. If user diligently
+ * satisified these conditions, pass the size through.
+ */
+ if ((sz % page_sz) == 0 && is_pow_of_2(sz / page_sz))
+ return sz;
+
+ /* Otherwise find closest (page_sz * power_of_2) product bigger than
+ * user-set size to satisfy both user size request and kernel
+ * requirements and substitute correct max_entries for map creation.
+ */
+ for (mul = 1; mul <= UINT_MAX / page_sz; mul <<= 1) {
+ if (mul * page_sz > sz)
+ return mul * page_sz;
+ }
+
+ /* if it's impossible to satisfy the conditions (i.e., user size is
+ * very close to UINT_MAX but is not a power-of-2 multiple of
+ * page_size) then just return original size and let kernel reject it
+ */
+ return sz;
+}
+
static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def)
{
map->def.type = def->map_type;
@@ -2333,6 +2364,10 @@ static void fill_map_from_def(struct bpf_map *map, const struct btf_map_def *def
map->btf_key_type_id = def->key_type_id;
map->btf_value_type_id = def->value_type_id;
+ /* auto-adjust BPF ringbuf map max_entries to be a multiple of page size */
+ if (map->def.type == BPF_MAP_TYPE_RINGBUF)
+ map->def.max_entries = adjust_ringbuf_sz(map->def.max_entries);
+
if (def->parts & MAP_DEF_MAP_TYPE)
pr_debug("map '%s': found type = %u.\n", map->name, def->map_type);
@@ -4306,9 +4341,15 @@ struct bpf_map *bpf_map__inner_map(struct bpf_map *map)
int bpf_map__set_max_entries(struct bpf_map *map, __u32 max_entries)
{
- if (map->fd >= 0)
+ if (map->obj->loaded)
return libbpf_err(-EBUSY);
This change is not explained in the commit message. Could you explain
why this change? In libbpf.c, there are multiple places using map->f >=
0, not sure whether there are any potential issues for those cases or not.
+
map->def.max_entries = max_entries;
+
+ /* auto-adjust BPF ringbuf map max_entries to be a multiple of page size */
+ if (map->def.type == BPF_MAP_TYPE_RINGBUF)
+ map->def.max_entries = adjust_ringbuf_sz(map->def.max_entries);
+
return 0;
}
[...]
