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

[Mina Almasry]

On Thu, Nov 9, 2023 at 12:30 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>
> I'm trying to wrap my head around the whole infra... the above line is
> confusing. Why do you increment dma_addr? it will be re-initialized in
> the next iteration.
>

That is just a mistake, sorry. Will remove this increment.

On Thu, Nov 9, 2023 at 1:29 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:> >>>
> >>> 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?
>
> I seems to misunderstand it. In that case, it seems to be about
> defensive programming like other checking.
>
> By looking at it more closely, it seems napi_frag_unref() call
> page_pool_page_put_many() directly， which means devmem seems to
> be bypassing the napi_safe optimization.
>
> Can napi_frag_unref() reuse napi_pp_put_page() in order to reuse
> the napi_safe optimization?
>

I think it already does. page_pool_page_put_many() is only called if
!recycle or !napi_pp_put_page(). In that case
page_pool_page_put_many() is just a replacement for put_page(),
because this 'page' may be an iov.

-- 
Thanks,
Mina