RE: [PATCH net,v2] net: mana: Fix RX skb truesize accounting

[Haiyang Zhang]

> -----Original Message-----
> From: Dipayaan Roy <dipayanroy@xxxxxxxxxxxxxxxxxxx>
> Sent: Tuesday, March 24, 2026 2:14 PM
> To: KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; wei.liu@xxxxxxxxxx; Dexuan Cui
> <DECUI@xxxxxxxxxxxxx>; andrew+netdev@xxxxxxx; davem@xxxxxxxxxxxxx;
> edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx; leon@xxxxxxxxxx;
> Long Li <longli@xxxxxxxxxxxxx>; Konstantin Taranov
> <kotaranov@xxxxxxxxxxxxx>; horms@xxxxxxxxxx;
> shradhagupta@xxxxxxxxxxxxxxxxxxx; ssengar@xxxxxxxxxxxxxxxxxxx;
> ernis@xxxxxxxxxxxxxxxxxxx; Shiraz Saleem <shirazsaleem@xxxxxxxxxxxxx>;
> linux-hyperv@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-rdma@xxxxxxxxxxxxxxx;
> stephen@xxxxxxxxxxxxxxxxxx; jacob.e.keller@xxxxxxxxx; Dipayaan Roy
> <dipayanroy@xxxxxxxxxxxxx>
> Subject: [PATCH net,v2] net: mana: Fix RX skb truesize accounting
> 
> MANA passes rxq->alloc_size to napi_build_skb() for all RX buffers.
> It is correct for fragment-backed RX buffers, where alloc_size matches
> the actual backing allocation used for each packet buffer. However, in
> the non-fragment RX path mana allocates a full page, or a higher-order
> page, per RX buffer. In that case alloc_size only reflects the usable
> packet area and not the actual backing memory.
> 
> This causes napi_build_skb() to underestimate the skb backing allocation
> in the single-buffer RX path, so skb->truesize is derived from a value
> smaller than the real RX buffer allocation.
> 
> Fix this by updating alloc_size in the non-fragment RX path to the
> actual backing allocation size before it is passed to napi_build_skb().
> 
> Fixes: 730ff06d3f5c ("net: mana: Use page pool fragments for RX buffers
> instead of full pages to improve memory efficiency.")
> Signed-off-by: Dipayaan Roy <dipayanroy@xxxxxxxxxxxxxxxxxxx>

Reviewed-by: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>

> ---
> Changes in v2:
>  - Added maintainers missed in v1.
> ---
>  drivers/net/ethernet/microsoft/mana/mana_en.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c
> b/drivers/net/ethernet/microsoft/mana/mana_en.c
> index ea71de39f996..884f8e548174 100644
> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
> @@ -766,6 +766,13 @@ static void mana_get_rxbuf_cfg(struct
> mana_port_context *apc,
>  		}
> 
>  		*frag_count = 1;
> +
> +		/* In the single-buffer path, napi_build_skb() must see the
> +		 * actual backing allocation size so skb->truesize reflects
> +		 * the full page (or higher-order page), not just the usable
> +		 * packet area.
> +		 */
> +		*alloc_size = PAGE_SIZE << get_order(*alloc_size);
>  		return;
>  	}
> 
> --
> 2.43.0
>