Subject: Re: [PATCH net-next v8 11/17] io_uring/zcrx: implement zerocopy receive pp memory provider

From: Jakub Kicinski <[email-masked]>
Date: Mon, 9 Dec 2024 20:01:56 -0800

On Wed,  4 Dec 2024 09:21:50 -0800 David Wei wrote:
> Then, either the buffer is dropped and returns back to the page pool
> into the ->freelist via io_pp_zc_release_netmem, in which case the page
> pool will match hold_cnt for us with ->pages_state_release_cnt. Or more
> likely the buffer will go through the network/protocol stacks and end up
> in the corresponding socket's receive queue. From there the user can get
> it via an new io_uring request implemented in following patches. As
> mentioned above, before giving a buffer to the user we bump the refcount
> by IO_ZC_RX_UREF.
> 
> Once the user is done with the buffer processing, it must return it back
> via the refill queue, from where our ->alloc_netmems implementation can
> grab it, check references, put IO_ZC_RX_UREF, and recycle the buffer if
> there are no more users left. As we place such buffers right back into
> the page pools fast cache and they didn't go through the normal pp
> release path, they are still considered "allocated" and no pp hold_cnt
> is required. For the same reason we dma sync buffers for the device
> in io_zc_add_pp_cache().

Can you say more about the IO_ZC_RX_UREF bias? net_iov is not the page
struct, we can add more fields. In fact we have 8B of padding in it
that can be allocated without growing the struct. So why play with
biases? You can add a 32b atomic counter for how many refs have been
handed out to the user.