On Mon, Aug 17, 2020 at 12:12:12PM -0700, Cong Wang wrote:

So netdev people will have to understand and support PageSlab() or
page_count()?

Yes.  As they came up with that contrived rule what is acceptable
for sendpage.  No one else really knows and other subsystems like the
block layer are perfectly fine with it.

If it is unusual even for mm people, how could netdev people suppose
to understand this unusual mm bug? At least not any better.

It is not a mm bug, it is a networking quirk.