Re: [PATCH net 1/2] net/devmem: Reject insufficiently large dmabuf pools

[Stanislav Fomichev <stfomichev@gmail.com>]

On 04/23, Cosmin Ratiu wrote:
> Drivers that are told to allocate RX buffers from pools of DMA memory
> should have enough memory in the pool to satisfy projected allocation
> requests (a function of ring size, MTU & other parameters). If there's
> not enough memory, RX ring refill might fail later at inconvenient times
> (e.g. during NAPI poll).
> 
> This commit adds a check at dmabuf pool init time that compares the
> amount of memory in the underlying chunk pool (configured by the user
> space application providing dmabuf memory) with the desired pool size
> (previously set by the driver) and fails with an error message if chunk
> memory isn't enough.
> 
> Fixes: 0f9214046893 ("memory-provider: dmabuf devmem memory provider")
> Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
> ---
>  net/core/devmem.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 6e27a47d0493..651cd55ebb28 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -299,6 +299,7 @@ net_devmem_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,
>  int mp_dmabuf_devmem_init(struct page_pool *pool)
>  {
>  	struct net_devmem_dmabuf_binding *binding = pool->mp_priv;
> +	size_t size;
>  
>  	if (!binding)
>  		return -EINVAL;
> @@ -312,6 +313,16 @@ int mp_dmabuf_devmem_init(struct page_pool *pool)
>  	if (pool->p.order != 0)
>  		return -E2BIG;
>  
> +	/* Validate that the underlying dmabuf has enough memory to satisfy
> +	 * requested pool size.
> +	 */

I think it's useful to have a check, but note that this check is in no
way a guarantee that the genpool has enough capacity. We can use the
same binding on multiple queues... Can you expand the comment a bit
to explain that it's more of a sanity check than a guarantee?

> +	size = gen_pool_size(binding->chunk_pool) >> PAGE_SHIFT;
> +	if (size < pool->p.pool_size) {
> +		pr_warn("%s: Insufficient dmabuf memory (%zu pages) to satisfy pool_size (%u pages)\n",

Let's print the sizes in bytes? We might have order>0 pages soon in the
pp: https://lore.kernel.org/netdev/20250421222827.283737-1-kuba@kernel.org/T/#t