Re: [PATCH net-next 1/6] xdp: introduce mb in xdp_buff/xdp_frame

On Sun, 23 Aug 2020 17:08:30 +0300
Shay Agroskin [off-list ref] wrote:

quoted

diff --git a/include/net/xdp.h b/include/net/xdp.h
index 3814fb631d52..42f439f9fcda 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h

@@ -72,7 +72,8 @@ struct xdp_buff {
 	void *data_hard_start;
 	struct xdp_rxq_info *rxq;
 	struct xdp_txq_info *txq;
-	u32 frame_sz; /* frame size to deduce 

data_hard_end/reserved tailroom*/
+	u32 frame_sz:31; /* frame size to deduce 
data_hard_end/reserved tailroom*/
+	u32 mb:1; /* xdp non-linear buffer */
 };
 
 /* Reserve memory area at end-of data area.

@@ -96,7 +97,8 @@ struct xdp_frame {
 	u16 len;
 	u16 headroom;
 	u32 metasize:8;
-	u32 frame_sz:24;
+	u32 frame_sz:23;
+	u32 mb:1; /* xdp non-linear frame */  

Although this issue wasn't introduced with this patch, why not 
make frame_sz field to be the same size in xdp_buff and xdp_frame 
?

This is all about struct layout and saving memory size, due to
cacheline access. Please read up on this and use the tool pahole to
inspect the struct memory layout.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer