Re: [PATCH bpf-next] bpf: Shrink size of struct bpf_map/bpf_array.

[Yonghong Song <yonghong.song@linux.dev>]

On 2/20/24 3:50 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> 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@kernel.org>

Acked-by: Yonghong Song <yonghong.song@linux.dev>