On Mon, Jan 05, 2009 at 09:36:14AM +0100, Johannes Berg wrote:
> On Sun, 2009-01-04 at 19:41 +0100, Andi Kleen wrote:
>
> > I think most adjustments are too small to be noticed. Typically
> > they are just for a few bytes in the header. truesize
> > is already larger, so it can tolerate some slag.
>
> This statement is incompatible with your patch when you think about the
> exact definition of truesize and the (unconditional!) adjustments your
> patch makes.
__alloc_skb does
size = SKB_DATA_ALIGN(size);
skb->truesize = size + sizeof(struct sk_buff);
[ BTW it would be probably better if alloc_skb() just asked
slab what the truesize is for kmalloc instead of guessing wrong like this.
But that's a different topic]
and SKB_DATA_ALIGN is
#define SKB_DATA_ALIGN(X) (((X) + (SMP_CACHE_BYTES - 1)) & \
~(SMP_CACHE_BYTES - 1))
and on my configuration SMP_CACHE_BYTES is 64
skb_truesize_check does
int len = sizeof(struct sk_buff) + skb->len;
if (unlikely((int)skb->truesize < len))
skb_truesize_bug(skb);
This means if the change is less than the 64byte cache alignment
(or more commonly 128 bytes on GENERIC_CPU distro kernels)
it won't be reported. To my knowledge header adjustments are usually
smaller and that is what pskb_expand_head() is usually used for.
> > I also only see it occasionally (maybe 5-10 times/day) when
> > the wireless stack appends a lot of data.
>
> Except the data it appends should generally be of the same or very
> similar size under unchanging conditions, so that doesn't make a lot of
> sense either.
I don't know too much about the packet dynamics of the wireless stack,
but I can only report what my machine printed out.
Here are some excerpts:
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=769, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1404, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=769, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=920, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=462, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1452, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1452, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1452, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1452, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1452, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=468, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=820, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=820, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=468, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=533, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=468, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=542, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (600) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1349, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
SKB BUG: Invalid truesize (920) len=1440, sizeof(sk_buff)=216
> I disagree, obviously. I knew there was some truesize corruption, and I
> think you for tracing down where it occurs. I'll investigate a proper
> fix when I get around to that, meanwhile I don't think the problem is
> awfully urgent since we've had this going on for quite a while and, if
> any, it probably only affects/corrupts the raw monitor sockets.
I didn't use any monitoring with this. No tcpdump, no wireless
sniffer tools or anything. It happened all the time during
normal operation.
-Andi
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