From: Alexei Starovoitov <ast@xxxxxxxxxx>
Back in 2018 the commit be95a845cc44 ("bpf: avoid false sharing of map refcount with max_entries")
added ____cacheline_aligned to "struct bpf_map" to make sure that fields like
refcnt don't share a cache line with max_entries that is used to bounds check
map access. That was done to make spectre style attacks harder. The main
mitigation is done via code similar to array_index_nospec(), of course.
This was an additional precaution.
It increased the size of "struct bpf_map" a little, but it's affect
on all other maps (like array) is significant, since "struct bpf_map" is
typically the first member in other map types.
Undo this ____cacheline_aligned tag. Instead move freeze_mutex field around,
so that refcnt and max_entries are still in different cache lines.
The main effect is seen in sizeof(struct bpf_array) that reduces from 320 to 248 bytes.
BEFORE:
struct bpf_map {
const struct bpf_map_ops * ops; /* 0 8 */
...
char name[16]; /* 96 16 */
/* XXX 16 bytes hole, try to pack */
/* --- cacheline 2 boundary (128 bytes) --- */
atomic64_t refcnt __attribute__((__aligned__(64))); /* 128 8 */
...
/* size: 256, cachelines: 4, members: 30 */
/* sum members: 232, holes: 1, sum holes: 16 */
/* padding: 8 */
/* paddings: 1, sum paddings: 2 */
} __attribute__((__aligned__(64)));
struct bpf_array {
struct bpf_map map; /* 0 256 */
...
/* size: 320, cachelines: 5, members: 5 */
/* padding: 48 */
/* paddings: 1, sum paddings: 8 */
} __attribute__((__aligned__(64)));
AFTER:
struct bpf_map {
/* size: 232, cachelines: 4, members: 30 */
/* paddings: 1, sum paddings: 2 */
/* last cacheline: 40 bytes */
};
struct bpf_array {
/* size: 248, cachelines: 4, members: 5 */
/* last cacheline: 56 bytes */
};
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
---
include/linux/bpf.h | 12 +++---------
1 file changed, 3 insertions(+), 9 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index c7aa99b44dbd..814dc913a968 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -251,10 +251,7 @@ struct bpf_list_node_kern {
} __attribute__((aligned(8)));
struct bpf_map {
- /* The first two cachelines with read-mostly members of which some
- * are also accessed in fast-path (e.g. ops, max_entries).
- */
- const struct bpf_map_ops *ops ____cacheline_aligned;
+ const struct bpf_map_ops *ops;
struct bpf_map *inner_map_meta;
#ifdef CONFIG_SECURITY
void *security;
@@ -276,17 +273,14 @@ struct bpf_map {
struct obj_cgroup *objcg;
#endif
char name[BPF_OBJ_NAME_LEN];
- /* The 3rd and 4th cacheline with misc members to avoid false sharing
- * particularly with refcounting.
- */
- atomic64_t refcnt ____cacheline_aligned;
+ struct mutex freeze_mutex;
+ atomic64_t refcnt;
atomic64_t usercnt;
/* rcu is used before freeing and work is only used during freeing */
union {
struct work_struct work;
struct rcu_head rcu;
};
- struct mutex freeze_mutex;
atomic64_t writecnt;
/* 'Ownership' of program-containing map is claimed by the first program
* that is going to use this map or by the first program which FD is
--
2.34.1
