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Commit a02eb473 authored by Paul Durrant's avatar Paul Durrant Committed by David S. Miller
Browse files

xen-netback: worse-case estimate in xenvif_rx_action is underestimating 



The worse-case estimate for skb ring slot usage in xenvif_rx_action()
fails to take fragment page_offset into account. The page_offset does,
however, affect the number of times the fragmentation code calls
start_new_rx_buffer() (i.e. consume another slot) and the worse-case
should assume that will always return true. This patch adds the page_offset
into the DIV_ROUND_UP for each frag.

Unfortunately some frontends aggressively limit the number of requests
they post into the shared ring so to avoid an estimate that is 'too'
pessimal it is capped at MAX_SKB_FRAGS.

Signed-off-by: default avatarPaul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0576eddf

drivers/net/xen-netback/netback.c

+20 −1
Original line number Diff line number Diff line
@@ -493,9 +493,28 @@ static void xenvif_rx_action(struct xenvif *vif) 
						PAGE_SIZE);

		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {

			unsigned int size;

			unsigned int offset;



			size = skb_frag_size(&skb_shinfo(skb)->frags[i]);

			max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE);

			offset = skb_shinfo(skb)->frags[i].page_offset;



			/* For a worse-case estimate we need to factor in

			 * the fragment page offset as this will affect the

			 * number of times xenvif_gop_frag_copy() will

			 * call start_new_rx_buffer().

			 */

			max_slots_needed += DIV_ROUND_UP(offset + size,

							 PAGE_SIZE);

		}



		/* To avoid the estimate becoming too pessimal for some

		 * frontends that limit posted rx requests, cap the estimate

		 * at MAX_SKB_FRAGS.

		 */

		if (max_slots_needed > MAX_SKB_FRAGS)

			max_slots_needed = MAX_SKB_FRAGS;



		/* We may need one more slot for GSO metadata */

		if (skb_is_gso(skb) &&

		   (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||

		    skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6))