Re: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory limits

On 6/17/19 8:53 PM, Christoph Paasch wrote:

On Mon, Jun 17, 2019 at 8:44 PM Eric Dumazet [off-list ref] wrote:

quoted

On 6/17/19 8:19 PM, Christoph Paasch wrote:

quoted

Yes, this does the trick for my packetdrill-test.

I wonder, is there a way we could end up in a situation where we can't
retransmit anymore?
For example, sk_wmem_queued has grown so much that the new test fails.
Then, if we legitimately need to fragment in __tcp_retransmit_skb() we
won't be able to do so. So we will never retransmit. And if no ACK
comes back in to make some room we are stuck, no?

Well, RTO will eventually fire.

But even the RTO would have to go through __tcp_retransmit_skb(), and
let's say the MTU of the interface changed and thus we need to
fragment. tcp_fragment() would keep on failing then, no? Sure,
eventually we will ETIMEOUT but that's a long way to go.

Also I want to point that normal skb split for not-yet transmitted skbs
does not use tcp_fragment(), with one exception (the one you hit)

Only the first skb in write queue can possibly have payload in skb->head
and might go through tcp_fragment()

Other splits will use tso_fragment() which does not enforce sk_wmem_queued limits (yet)

So things like TLP should work.