On Fri, Nov 27, 2020 at 7:26 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > On Thu, 26 Nov 2020 at 17:35, Willem de Bruijn > <willemdebruijn.kernel@xxxxxxxxx> wrote: > > On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@xxxxxxxxxx> wrote: > [...] > > > 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(). > > > 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.
