Re: [PATCH net v1 2/2] gve: use max allowed ring size for ZC page_pools

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/7/25 13:35, Dragos Tatulea wrote:
> On Thu, Nov 06, 2025 at 05:18:33PM -0800, Jakub Kicinski wrote:
>> On Thu, 6 Nov 2025 17:25:43 +0000 Dragos Tatulea wrote:
>>> On Wed, Nov 05, 2025 at 06:56:46PM -0800, Mina Almasry wrote:
>>>> On Wed, Nov 5, 2025 at 6:22 PM Jakub Kicinski <kuba@kernel.org> wrote:
>>>>> Increasing cache sizes to the max seems very hacky at best.
>>>>> The underlying implementation uses genpool and doesn't even
>>>>> bother to do batching.
>>>>
>>>> OK, my bad. I tried to think through downsides of arbitrarily
>>>> increasing the ring size in a ZC scenario where the underlying memory
>>>> is pre-pinned and allocated anyway, and I couldn't think of any, but I
>>>> won't argue the point any further.
>>>>    
>>> I see a similar issue with io_uring as well: for a 9K MTU with 4K ring
>>> size there are ~1% allocation errors during a simple zcrx test.
>>>
>>> mlx5 calculates 16K pages and the io_uring zcrx buffer matches exactly
>>> that size (16K * 4K). Increasing the buffer doesn't help because the
>>> pool size is still what the driver asked for (+ also the
>>> internal pool limit). Even worse: eventually ENOSPC is returned to the
>>> application. But maybe this error has a different fix.
>>
>> Hm, yes, did you trace it all the way to where it comes from?
>> page pool itself does not have any ENOSPC AFAICT. If the cache
>> is full we free the page back to the provider via .release_netmem
>>
> Yes I did. It happens in io_cqe_cache_refill() when there are no more
> CQEs:
> https://elixir.bootlin.com/linux/v6.17.7/source/io_uring/io_uring.c#L775

-ENOSPC here means io_uring's CQ got full. It's non-fatal, the user
is expected to process completions and reissue the request. And it's
best to avoid that for performance reasons, e.g. by making the CQ
bigger as you already noted.

-- 
Pavel Begunkov