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Re: [Xen-devel] [PATCH net 1/4] xen-netback: Fix handling frag_list on grant op error path

On 18/07/14 16:24, Wei Liu wrote:
On Thu, Jul 17, 2014 at 08:09:49PM +0100, Zoltan Kiss wrote:
The error handling for skb's with frag_list was completely wrong, it caused
double unmap attempts to happen if the error was on the first skb. Move it to
the right place in the loop.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Reported-by: Armin Zentai <armin.zentai@xxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
diff --git a/drivers/net/xen-netback/netback.c 
index 1844a47..604ff71 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1030,10 +1030,16 @@ static int xenvif_tx_check_gop(struct xenvif_queue 
        struct gnttab_map_grant_ref *gop_map = *gopp_map;
        u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
+       /* This points to the shinfo of the actually checked skb, which could be
+        * either the first or the one on the frag_list
+        */

I think "checked skb" should be "skb being checked". Feel free to
disagree as I'm not native English speaker. :-/

        struct skb_shared_info *shinfo = skb_shinfo(skb);
+       /* If this is non-NULL, we are currently checking the frag_list skb, and
+        * this points to the shinfo of the first one
+        */
+       struct skb_shared_info *first_shinfo = NULL;
        int nr_frags = shinfo->nr_frags;
        int i, err;
-       struct sk_buff *first_skb = NULL;

        /* Check status of header. */
        err = (*gopp_copy)->status;
@@ -1086,31 +1092,28 @@ check_frags:
                        xenvif_idx_unmap(queue, pending_idx);

+               /* And if we found the error while checking the frag_list, unmap
+                * the first skb's frags
+                */
+               if (first_shinfo) {
+                       for (j = 0; j < first_shinfo->nr_frags; j++) {
+                               pending_idx = 
+                               xenvif_idx_unmap(queue, pending_idx);
+                       }
+               }
                /* Remember the error: invalidate all subsequent fragments. */
                err = newerr;

-       if (skb_has_frag_list(skb)) {
-               first_skb = skb;
-               skb = shinfo->frag_list;
-               shinfo = skb_shinfo(skb);
+       if (skb_has_frag_list(skb) && !first_shinfo) {

Will it ever come to the point that we have another skb in this skb's
frag list? Is there any reason prevents you from looping over the
(possible) subsequent skbs? I guess if the error is deep in the list
it's a bit hard to bookkeep...

+               first_shinfo = skb_shinfo(skb);
+               shinfo = skb_shinfo(skb_shinfo(skb)->frag_list);

In that case I would suggest you add
BUG_ON(skb_has_frag_list(skb_shinfo(skb)->frag_list)). I think having
more nested frag_list should be a bug in current design.

There are already 3 things which prevents this
- in count_requests we drop the packet if it has more than XEN_NETBK_LEGACY_SLOTS_MAX slots - in get_requests there is a BUG_ON(frag_overflow > MAX_SKB_FRAGS), which shouldn't really due to the prev point
- in the same funciont we create a frag_list skb exactly once

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