On 05.02.26 11:54, Paolo Abeni wrote:

On 2/3/26 8:19 PM, Oliver Hartkopp wrote:

quoted

When CONFIG_CAN is enabled the skbuff_ext_cache element would increase
in size by 8 bytes (sizeof(struct can_skb_ext)) on my machine (see
pahole output below).

So when everything is enabled it would be

CONFIG_SKB_EXTENSIONS
8  bytes sizeof(struct skb_ext)
CONFIG_BRIDGE_NETFILTER
32 bytes sizeof(struct nf_bridge_info)
CONFIG_XFRM
88 bytes sizeof(struct sec_path)
CONFIG_NET_TC_SKB_EXT
16 bytes sizeof(struct tc_skb_ext)
CONFIG_MPTCP
32 bytes sizeof(struct mptcp_ext)
CONFIG_MCTP_FLOWS
8  bytes sizeof(struct mctp_flow)
CONFIG_INET_PSP
8  bytes sizeof(struct psp_skb_ext)
CONFIG_CAN
8  bytes sizeof(struct can_skb_ext)
---------
200 bytes total skbuff_ext_cache element size
(255 * 8 = 2040 bytes max space for skb extension users).

Does this answer your question?

Yes, thank you for the collaboration!

I think there is mistake above: sizeof(struct skb_ext) should be 16 when
more than 3 skb extensions are enabled.

Oh, yes. You are right!

struct skb_ext {
         refcount_t                 refcnt;               /*     0     4 */
         u8                         offset[7];            /*     4     7 */
         u8                         chunks;               /*    11     1 */

         /* XXX 4 bytes hole, try to pack */

         char                       data[] 
__attribute__((__aligned__(8))); /*    16     0 */

         /* size: 16, cachelines: 1, members: 4 */
         /* sum members: 12, holes: 1, sum holes: 4 */
         /* forced alignments: 1, forced holes: 1, sum forced holes: 4 */
         /* last cacheline: 16 bytes */
} __attribute__((__aligned__(8)));

After I enabled all kernel configs that would increase the skb 
extensions I just copied the missing pahole structs into the mail - not 
looking for the struct skb_ext again ...

that means that the total extension size already exceeds the 3
cachelines (192 bytes) boundary when all extensions are enabled and
adding the CAN one should not cause any regressions is such scenario.

Note that the "all skb extensions enabled" is possibly/likely NOT the
most common/relevant one, but I think there is some space we can squeeze
elsewhere if needed.

Right.

Many thanks!
Oliver