Re: [PATCH net 1/2] selftests: fix OOM in msg_zerocopy selftest

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 3 Jul 2024 19:32:33 -0700 Zijian Zhang wrote:
> > This test doesn't fail in netdev CI. Is the problem fix in net-next
> > somehow? Or the "always exits with OUT_OF_MEMORY" is an exaggerations?
> > (TBH I'm not even sure what it means to "exit with OUT_OF_MEMORY" in
> > this context.)
> >  
> The reason why this test doesn't fail in CI:
> 
> According to the test output,
> # ipv4 tcp -z -t 1
> # tx=111332 (6947 MB) txc=111332 zc=n
> zerocopy is false here.
> 
> This is because of some limitation of zerocopy in localhost.
> Specifically, the subsection "Notification Latency" in the sendmsg
> zerocopy the paper.
> 
> In order to make "zc=y", we may need to update skb_orphan_frags_rx to
> the same as skb_orphan_frags, recompile the kernel, and run the test.
> 
> By OUT_OF_MEMORY I mean:
> 
> Each calling of sendmsg with zerocopy will allocate an skb with
> sock_omalloc. If users never recv the notifications but keep calling
> sendmsg with zerocopy. The send system call will finally return with
> -ENOMEM.
> 
> I hope this clarifies your confusion :)

It does, thanks!