On Fri, Nov 27, 2020 at 7:26 AM Marco Elver [off-list ref] wrote:

On Thu, 26 Nov 2020 at 17:35, Willem de Bruijn
[off-list ref] wrote:

quoted

On Thu, Nov 26, 2020 at 3:19 AM Marco Elver [off-list ref] wrote:

[...]

quoted

quoted

Will send v2.

Does it make more sense to revert the patch that added the extensions
and the follow-on fixes and add a separate new patch instead?

That doesn't work, because then we'll end up with a build-broken
commit in between the reverts and the new version, because mac80211
uses skb_get_kcov_handle().

quoted

If adding a new field to the skb, even if only in debug builds,
please check with pahole how it affects struct layout if you
haven't yet.

Without KCOV:

        /* size: 224, cachelines: 4, members: 72 */
        /* sum members: 217, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 32 bytes */

With KCOV:

        /* size: 232, cachelines: 4, members: 73 */
        /* sum members: 225, holes: 1, sum holes: 2 */
        /* sum bitfield members: 36 bits, bit holes: 2, sum bit holes: 4 bits */
        /* forced alignments: 2 */
        /* last cacheline: 40 bytes */

Thanks. defconfig leaves some symbols disabled, but manually enabling
them just fills a hole, so 232 is indeed the worst case allocation.

I recall a firm edict against growing skb, but I don't know of a
hard limit at exactly 224.

There is a limit at 2048 - sizeof(struct skb_shared_data) == 1728B
when using pages for two ETH_FRAME_LEN (1514) allocations.

This would leave 1728 - 1514 == 214B if also squeezing the skb itself
in with the same allocation.

But I have no idea if this is used anywhere. Certainly have no example
ready. And as you show, the previous default already is at 224.

If no one else knows of a hard limit at 224 or below, I suppose the
next technical limit is just 256 for kmem cache purposes.

My understanding was that skb_extensions was supposed to solve this
problem of extending the skb without growing the main structure. Not
for this patch, but I wonder if we can resolve the issues exposed here
and make usable in more conditions.