Re: [PATCH net-next v19 06/13] memory-provider: dmabuf devmem memory provider

Pavel Begunkov
2024-08-14T14:11:49Z

On 8/13/24 22:13, Mina Almasry wrote:
> Implement a memory provider that allocates dmabuf devmem in the form of
> net_iov.
>
> The provider receives a reference to the struct netdev_dmabuf_binding
> via the pool->mp_priv pointer. The driver needs to set this pointer for
> the provider in the net_iov.
>
> The provider obtains a reference on the netdev_dmabuf_binding which
> guarantees the binding and the underlying mapping remains alive until
> the provider is destroyed.
>
> Usage of PP_FLAG_DMA_MAP is required for this memory provide such that
> the page_pool can provide the driver with the dma-addrs of the devmem.
>
> Support for PP_FLAG_DMA_SYNC_DEV is omitted for simplicity & p.order !=
> 0.
>
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
>
> ---
>
> v19:
> - Add PP_FLAG_ALLOW_UNREADABLE_NETMEM flag. It serves 2 purposes, (a)
> it guards drivers that don't support unreadable netmem (net_iov
> backed) from accidentally getting exposed to it, and (b) drivers that
> wish to create header pools can unset it for that pool to force
> readable netmem.
> - Add page_pool_check_memory_provider, which verifies that the driver
> has created a page_pool with the expected configuration. This is used
> to report to the user if the mp configuration succeeded, and also
> verify that the driver is doing the right thing.
> - Don't reset niov->dma_addr on allocation/free.
>
> v17:
> - Use ASSERT_RTNL (Jakub)
>
> v16:
> - Add DEBUG_NET_WARN_ON_ONCE(!rtnl_is_locked()), to catch cases if
> page_pool_init without rtnl_locking when the queue is provided. In
> this case, the queue configuration may be changed while we're initing
> the page_pool, which could be a race.
>
> v13:
> - Return on warning (Pavel).
> - Fixed pool->recycle_stats not being freed on error (Pavel).
> - Applied reviewed-by from Pavel.
>
> v11:
> - Rebase to not use the ops. (Christoph)
>
> v8:
> - Use skb_frag_size instead of frag->bv_len to fix patch-by-patch build
> error
>
> v6:
> - refactor new memory provider functions into net/core/devmem.c (Pavel)
>
> v2:
> - Disable devmem for p.order != 0
>
> v1:
> - static_branch check in page_is_page_pool_iov() (Willem & Paolo).
> - PP_DEVMEM -> PP_IOV (David).
> - Require PP_FLAG_DMA_MAP (Jakub).
>
...
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 301f4250ca82..2f2a7f4dee4c 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -17,6 +17,7 @@
> #include <linux/genalloc.h>
> #include <linux/dma-buf.h>
> #include <net/devmem.h>
> +#include <net/mp_dmabuf_devmem.h>
> #include <net/netdev_queues.h>
>
> #include "page_pool_priv.h"
> @@ -153,6 +154,10 @@ int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
> if (err)
> goto err_xa_erase;
>
> + err = page_pool_check_memory_provider(dev, rxq, binding);

Frankly, I pretty much don't like it.

1. We do it after reconfiguring the queue just to fail and reconfigure
it again.

2. It should be a part of the common path like netdev_rx_queue_restart(),
not specific to devmem TCP.

These two can be fixed by moving the check into
netdev_rx_queue_restart() just after ->ndo_queue_mem_alloc, assuming
that the callback where we init page pools.

3. That implicit check gives me bad feeling, instead of just getting
direct feedback from the driver, either it's a flag or an error
returned, we have to try to figure what exactly the driver did, with
a high chance this inference will fail us at some point.

And page_pool_check_memory_provider() is not that straightforward,
it doesn't walk through pools of a queue. Not looking too deep,
but it seems like the nested loop can be moved out with the same
effect, so it first looks for a pool in the device and the follows
with the bound_rxqs. And seems the bound_rxqs check would always turn
true, you set the binding into the map in
net_devmem_bind_dmabuf_to_queue() before the restart and it'll be there
after restart for page_pool_check_memory_provider(). Maybe I missed
something, but it's not super clear.

4. And the last thing Jakub mentioned is that we need to be prepared
to expose a flag to the userspace for whether a queue supports
netiov. Not really doable in a sane manner with such implicit
post configuration checks.

And that brings us back to the first approach I mentioned, where
we have a flag in the queue structure, drivers set it, and
netdev_rx_queue_restart() checks it before any callback. That's
where the thread with Jakub stopped, and it reads like at least
he's not against the idea.

> + if (err)
> + goto err_xa_erase;
> +
> return 0;
>
> err_xa_erase:
> @@ -305,4 +310,69 @@ void dev_dmabuf_uninstall(struct net_device *dev)
> xa_erase(&binding->bound_rxqs, xa_idx);
> }
> }
> +
...
> diff --git a/net/core/page_pool_user.c b/net/core/page_pool_user.c
> index 3a3277ba167b..cbc54ee4f670 100644
> --- a/net/core/page_pool_user.c
> +++ b/net/core/page_pool_user.c
> @@ -344,6 +344,32 @@ void page_pool_unlist(struct page_pool *pool)
> mutex_unlock(&page_pools_lock);
> }
>
> +int page_pool_check_memory_provider(struct net_device *dev,
> + struct netdev_rx_queue *rxq,
> + struct net_devmem_dmabuf_binding *binding)
> +{
> + struct netdev_rx_queue *binding_rxq;
> + struct page_pool *pool;
> + struct hlist_node *n;
> + unsigned long xa_idx;
> +
> + mutex_lock(&page_pools_lock);
> + hlist_for_each_entry_safe(pool, n, &dev->page_pools, user.list) {
> + if (pool->mp_priv != binding)
> + continue;
> +
> + xa_for_each(&binding->bound_rxqs, xa_idx, binding_rxq) {
> + if (rxq != binding_rxq)
> + continue;
> +
> + mutex_unlock(&page_pools_lock);
> + return 0;
> + }
> + }
> + mutex_unlock(&page_pools_lock);
> + return -ENODATA;
> +}
> +
> static void page_pool_unreg_netdev_wipe(struct net_device *netdev)
> {
> struct page_pool *pool;

--
Pavel Begunkov