linux-kernel - Re: [PATCH net v1 2/2] gve: use max allowed ring size for ZC page

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Message-ID: <fdumvcrk5bcngulfbjxidtezrdifi6kzouea3ygr5rq5hrzdfv@mum2kdsiqkmz>
Date: Mon, 10 Nov 2025 12:48:58 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Joshua Washington <joshwash@...gle.com>, 
	Harshitha Ramamurthy <hramamurthy@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, Simon Horman <horms@...nel.org>, 
	Willem de Bruijn <willemb@...gle.com>, ziweixiao@...gle.com, Vedant Mathur <vedantmathur@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH net v1 2/2] gve: use max allowed ring size for ZC
 page_pools

On Mon, Nov 10, 2025 at 12:36:46PM +0000, Pavel Begunkov wrote:
> 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@...nel.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.
Got it. Thanks Pavel!

Thanks,
Dragos