linux-kernel - Re: [PATCH 8/9] mlx5: stop warning for 64KB pages

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Message-ID: <CAFhGd8pTdJNzmxhUaCpDkWpYiAP0Gvm6K=o+w5uE-g20M7u3rA@mail.gmail.com>
Date: Thu, 28 Mar 2024 15:09:30 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>, 
	Leon Romanovsky <leon@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Nathan Chancellor <nathan@...nel.org>, Jonathan Lemon <jonathan.lemon@...il.com>, 
	Maxim Mikityanskiy <maxtram95@...il.com>, Daniel Borkmann <daniel@...earbox.net>, 
	Arnd Bergmann <arnd@...db.de>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Tariq Toukan <tariqt@...dia.com>, Gal Pressman <gal@...dia.com>, 
	netdev@...r.kernel.org, linux-rdma@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 8/9] mlx5: stop warning for 64KB pages

On Thu, Mar 28, 2024 at 7:32 AM Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> When building with 64KB pages, clang points out that xsk->chunk_size
> can never be PAGE_SIZE:

This is under W=1 right? Otherwise this is a mighty annoying warning.

>
> 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@kernelorg/
> Link: https://lore.kernel.org/lkml/a7b27541-0ebb-4f2d-bd06-270a4d404613@app.fastmail.com/
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Justin Stitt <justinstitt@...gle.com>

> ---
>  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 06592b9f0424..9240cfe25d10 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xsk/setup.c
> @@ -28,8 +28,10 @@ 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 || xsk->chunk_size < MLX5E_MIN_XSK_CHUNK_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) {
>                 mlx5_core_err(mdev, "XSK chunk size %u out of bounds [%u, %lu]\n", xsk->chunk_size,
>                               MLX5E_MIN_XSK_CHUNK_SIZE, PAGE_SIZE);
>                 return false;
> --
> 2.39.2
>