linux-kernel - RE: [PATCH] [v2] mlx5: stop warning for 64KB pages

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Date:   Wed, 20 Oct 2021 08:39:56 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Leon Romanovsky <leon@...nel.org>
CC:     Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Aya Levin <ayal@...dia.com>,
        "Eran Ben Elisha" <eranbe@...dia.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [PATCH] [v2] mlx5: stop warning for 64KB pages

From: Arnd Bergmann
> Sent: 15 October 2021 16:21
> 
> From: Arnd Bergmann <arnd@...db.de>
> 
> When building with 64KB pages, clang points out that xsk->chunk_size
> can never be PAGE_SIZE:
> 
> drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c:19:22: error: result of comparison of constant
> 65536 with expression of type 'u16' (aka 'unsigned short') is always false [-Werror,-Wtautological-
> constant-out-of-range-compare]
>         if (xsk->chunk_size > PAGE_SIZE ||
>             ~~~~~~~~~~~~~~~ ^ ~~~~~~~~~
> 
> In older versions of this code, using PAGE_SIZE was the only
> possibility, so this would have never worked on 64KB page kernels,
> but the patch apparently did not address this case completely.
> 
> As Maxim Mikityanskiy suggested, 64KB chunks are really not all that
> useful, so just shut up the warning by adding a cast.
> 
> Fixes: 282c0c798f8e ("net/mlx5e: Allow XSK frames smaller than a page")
> Link: https://lore.kernel.org/netdev/20211013150232.2942146-1-arnd@kernel.org/
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> index 538bc2419bd8..228257010f32 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> @@ -15,8 +15,10 @@ bool mlx5e_validate_xsk_param(struct mlx5e_params *params,
>  			      struct mlx5e_xsk_param *xsk,
>  			      struct mlx5_core_dev *mdev)
>  {
> -	/* AF_XDP doesn't support frames larger than PAGE_SIZE. */
> -	if (xsk->chunk_size > PAGE_SIZE ||
> +	/* AF_XDP doesn't support frames larger than PAGE_SIZE,
> +	 * and xsk->chunk_size is limited to 65535 bytes.
> +	 */
> +	if ((size_t)xsk->chunk_size > PAGE_SIZE ||
>  			xsk->chunk_size < MLX5E_MIN_XSK_CHUNK_SIZE)
>  		return false;

How much smaller does the kernel get if you change 'chunk_size' from
_u16 to 'unsigned int'. ?
Especially for a non-x86 build?
Or is it a hardware constrained size??

	David

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