On Wed, Jul 10, 2024 at 9:37 AM Jakub Kicinski kuba@kernel.org wrote:
On Wed, 10 Jul 2024 00:17:37 +0000 Mina Almasry wrote:
net_devmem_dmabuf_binding_get(binding);Why does every iov need to hold a ref? pp holds a ref and does its own accounting, so it won't disappear unless all the pages are returned.
I guess it doesn't really need to, but this is the design/approach I went with, and I actually prefer it a bit. The design is borrowed from how struct dev_pagemap does this, IIRC. Every page allocated from the pgmap holds a reference to the pgmap to ensure the pgmap doesn't go away while some page that originated from it is out in the wild, and similarly I did so in the binding here.
We could assume that the page_pool is accounting iovs for us, but that is not always true, right? page_pool_return_page() disconnects a netmem from the page_pool and AFAIU the page_pool can go away while there is such a netmem still in use in the net stack. Currently this can't happen with iovs because I currently don't support non-pp refcounting for iovs (so they're always recyclable), but you have a comment on the other patch asking why that works; depending on how we converge on that conversation, the details of how the pp refcounting could change.
It's nice to know that the binding refcounting will work regardless of the details of how the pp refcounting works. IMHO having the binding rely on the pp refcounting to ensure all the iovs are freed introduces some fragility.
Additionally IMO the net_devmem_dmabuf_binding_get/put aren't so expensive to want to optimize out, right? The allocation is a slow path anyway and the fast path recycles netmem.
-- Thanks, Mina