linux-kernel - Re: [PATCH net-next v4 08/11] net/mlx5e: Add support for UNREADABLE netmem page pools

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Message-ID: <6nd3d7z5dmpzpegbwfkhszmtjqmsb4af5ts36mpv5m6jfavo23@lwijppu24jjf>
Date: Thu, 12 Jun 2025 08:46:44 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Mina Almasry <almasrymina@...gle.com>, Mark Bloch <mbloch@...dia.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>, saeedm@...dia.com, 
	gal@...dia.com, leonro@...dia.com, tariqt@...dia.com, 
	Leon Romanovsky <leon@...nel.org>, netdev@...r.kernel.org, linux-rdma@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Cosmin Ratiu <cratiu@...dia.com>
Subject: Re: [PATCH net-next v4 08/11] net/mlx5e: Add support for UNREADABLE
 netmem page pools

On Wed, Jun 11, 2025 at 10:16:18PM -0700, Mina Almasry wrote:
> On Tue, Jun 10, 2025 at 8:20 AM Mark Bloch <mbloch@...dia.com> wrote:
> >
> > From: Saeed Mahameed <saeedm@...dia.com>
> >
> > On netdev_rx_queue_restart, a special type of page pool maybe expected.
> >
> > In this patch declare support for UNREADABLE netmem iov pages in the
> > pool params only when header data split shampo RQ mode is enabled, also
> > set the queue index in the page pool params struct.
> >
> > Shampo mode requirement: Without header split rx needs to peek at the data,
> > we can't do UNREADABLE_NETMEM.
> >
> > The patch also enables the use of a separate page pool for headers when
> > a memory provider is installed for the queue, otherwise the same common
> > page pool continues to be used.
> >
> > Signed-off-by: Saeed Mahameed <saeedm@...dia.com>
> > Reviewed-by: Dragos Tatulea <dtatulea@...dia.com>
> > Signed-off-by: Cosmin Ratiu <cratiu@...dia.com>
> > Signed-off-by: Tariq Toukan <tariqt@...dia.com>
> > Signed-off-by: Mark Bloch <mbloch@...dia.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > index 5e649705e35f..a51e204bd364 100644
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
> > @@ -749,7 +749,9 @@ static void mlx5e_rq_shampo_hd_info_free(struct mlx5e_rq *rq)
> >
> >  static bool mlx5_rq_needs_separate_hd_pool(struct mlx5e_rq *rq)
> >  {
> > -       return false;
> > +       struct netdev_rx_queue *rxq = __netif_get_rx_queue(rq->netdev, rq->ix);
> > +
> > +       return !!rxq->mp_params.mp_ops;
> 
> This is kinda assuming that all future memory providers will return
> unreadable memory, which is not a restriction I have in mind... in
> theory there is nothing wrong with memory providers that feed readable
> pages. Technically the right thing to do here is to define a new
> helper page_pool_is_readable() and have the mp report to the pp if
> it's all readable or not.
>
The API is already there: page_pool_is_unreadable(). But it uses the
same logic...

However, having a pp level API is a bit limiting: as Cosmin pointed out,
mlx5 can't use it because it needs to know in advance if this page_pool
is for unreadable memory to correctly size the data page_pool (with or
without headers).

> But all this sounds like a huge hassle for an unnecessary amount of
> future proofing, so I guess this is fine.
> 
> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
> 
Thanks!
Dragos