Re: [RFC PATCH v3 04/12] netdev: support binding dma-buf to netdevice

[Mina Almasry]

On Tue, Nov 7, 2023 at 7:40 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2023/11/8 5:59, Mina Almasry wrote:
> > On Mon, Nov 6, 2023 at 11:46 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>
> >> On 2023/11/6 10:44, Mina Almasry wrote:
> >>> +
> >>> +void __netdev_devmem_binding_free(struct netdev_dmabuf_binding *binding)
> >>> +{
> >>> +     size_t size, avail;
> >>> +
> >>> +     gen_pool_for_each_chunk(binding->chunk_pool,
> >>> +                             netdev_devmem_free_chunk_owner, NULL);
> >>> +
> >>> +     size = gen_pool_size(binding->chunk_pool);
> >>> +     avail = gen_pool_avail(binding->chunk_pool);
> >>> +
> >>> +     if (!WARN(size != avail, "can't destroy genpool. size=%lu, avail=%lu",
> >>> +               size, avail))
> >>> +             gen_pool_destroy(binding->chunk_pool);
> >>
> >>
> >> Is there any other place calling the gen_pool_destroy() when the above
> >> warning is triggered? Do we have a leaking for binding->chunk_pool?
> >>
> >
> > gen_pool_destroy BUG_ON() if it's not empty at the time of destroying.
> > Technically that should never happen, because
> > __netdev_devmem_binding_free() should only be called when the refcount
> > hits 0, so all the chunks have been freed back to the gen_pool. But,
> > just in case, I don't want to crash the server just because I'm
> > leaking a chunk... this is a bit of defensive programming that is
> > typically frowned upon, but the behavior of gen_pool is so severe I
> > think the WARN() + check is warranted here.
>
> It seems it is pretty normal for the above to happen nowadays because of
> retransmits timeouts, NAPI defer schemes mentioned below:
>
> https://lkml.kernel.org/netdev/168269854650.2191653.8465259808498269815.stgit@firesoul/
>
> And currently page pool core handles that by using a workqueue.

Forgive me but I'm not understanding the concern here.

__netdev_devmem_binding_free() is called when binding->ref hits 0.

binding->ref is incremented when an iov slice of the dma-buf is
allocated, and decremented when an iov is freed. So,
__netdev_devmem_binding_free() can't really be called unless all the
iovs have been freed, and gen_pool_size() == gen_pool_avail(),
regardless of what's happening on the page_pool side of things, right?

-- 
Thanks,
Mina