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

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

[Dipayaan Roy]

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@linux.microsoft.com>
---
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

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

[Haiyang Zhang]

> -----Original Message-----
> From: Dipayaan Roy <dipayanroy@linux.microsoft.com>
> Sent: Tuesday, March 24, 2026 2:14 PM
> To: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; wei.liu@kernel.org; Dexuan Cui
> <DECUI@microsoft.com>; andrew+netdev@lunn.ch; davem@davemloft.net;
> edumazet@google.com; kuba@kernel.org; pabeni@redhat.com; leon@kernel.org;
> Long Li <longli@microsoft.com>; Konstantin Taranov
> <kotaranov@microsoft.com>; horms@kernel.org;
> shradhagupta@linux.microsoft.com; ssengar@linux.microsoft.com;
> ernis@linux.microsoft.com; Shiraz Saleem <shirazsaleem@microsoft.com>;
> linux-hyperv@vger.kernel.org; netdev@vger.kernel.org; linux-
> kernel@vger.kernel.org; linux-rdma@vger.kernel.org;
> stephen@networkplumber.org; jacob.e.keller@intel.com; Dipayaan Roy
> <dipayanroy@microsoft.com>
> 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@linux.microsoft.com>

Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>

> ---
> 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
>