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Subject[PATCH net-next v5 6/9] xen-netback: Handle guests with too many frags
Date
Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
- create a new skb
- map the leftover slots to its frags (no linear buffer here!)
- chain it to the previous through skb_shinfo(skb)->frag_list
- map them
- copy the whole stuff into a brand new skb and send it to the stack
- unmap the 2 old skb's pages

v3:
- adding extra check for frag number
- consolidate alloc_skb's into xenvif_alloc_skb()
- BUG_ON(frag_overflow > MAX_SKB_FRAGS)

v4:
- handle error of skb_copy_expand()

v5:
- ratelimit error messages
- remove a tx_flags setting from xenvif_tx_submit

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>

---
drivers/net/xen-netback/netback.c | 124 ++++++++++++++++++++++++++++++++++---
1 file changed, 114 insertions(+), 10 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 22d05de..031258c 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -803,6 +803,20 @@ static inline void xenvif_tx_create_gop(struct xenvif *vif,
sizeof(*txp));
}

+static inline struct sk_buff *xenvif_alloc_skb(unsigned int size)
+{
+ struct sk_buff *skb =
+ alloc_skb(size + NET_SKB_PAD + NET_IP_ALIGN,
+ GFP_ATOMIC | __GFP_NOWARN);
+ if (unlikely(skb == NULL))
+ return NULL;
+
+ /* Packets passed to netif_rx() must have some headroom. */
+ skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
+
+ return skb;
+}
+
static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
struct sk_buff *skb,
struct xen_netif_tx_request *txp,
@@ -813,11 +827,16 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,
u16 pending_idx = *((u16 *)skb->data);
int start;
pending_ring_idx_t index;
- unsigned int nr_slots;
+ unsigned int nr_slots, frag_overflow = 0;

/* At this point shinfo->nr_frags is in fact the number of
* slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
*/
+ if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+ frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;
+ BUG_ON(frag_overflow > MAX_SKB_FRAGS);
+ shinfo->nr_frags = MAX_SKB_FRAGS;
+ }
nr_slots = shinfo->nr_frags;

/* Skip first skb fragment if it is on same page as header fragment. */
@@ -833,6 +852,30 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif *vif,

BUG_ON(shinfo->nr_frags > MAX_SKB_FRAGS);

+ if (frag_overflow) {
+ struct sk_buff *nskb = xenvif_alloc_skb(0);
+ if (unlikely(nskb == NULL)) {
+ if (net_ratelimit())
+ netdev_err(vif->dev,
+ "Can't allocate the frag_list skb.\n");
+ return NULL;
+ }
+
+ shinfo = skb_shinfo(nskb);
+ frags = shinfo->frags;
+
+ for (shinfo->nr_frags = 0; shinfo->nr_frags < frag_overflow;
+ shinfo->nr_frags++, txp++, gop++) {
+ index = pending_index(vif->pending_cons++);
+ pending_idx = vif->pending_ring[index];
+ xenvif_tx_create_gop(vif, pending_idx, txp, gop);
+ frag_set_pending_idx(&frags[shinfo->nr_frags],
+ pending_idx);
+ }
+
+ skb_shinfo(skb)->frag_list = nskb;
+ }
+
return gop;
}

@@ -846,6 +889,7 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
struct pending_tx_info *tx_info;
int nr_frags = shinfo->nr_frags;
int i, err, start;
+ struct sk_buff *first_skb = NULL;

/* Check status of header. */
err = gop->status;
@@ -866,6 +910,7 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
/* Skip first skb fragment if it is on same page as header fragment. */
start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);

+check_frags:
for (i = start; i < nr_frags; i++) {
int j, newerr;

@@ -900,11 +945,20 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
/* Not the first error? Preceding frags already invalidated. */
if (err)
continue;
-
/* First error: invalidate header and preceding fragments. */
- pending_idx = *((u16 *)skb->data);
- xenvif_idx_unmap(vif, pending_idx);
- xenvif_idx_release(vif, pending_idx, XEN_NETIF_RSP_OKAY);
+ if (!first_skb) {
+ pending_idx = *((u16 *)skb->data);
+ xenvif_idx_unmap(vif, pending_idx);
+ xenvif_idx_release(vif,
+ pending_idx,
+ XEN_NETIF_RSP_OKAY);
+ } else {
+ pending_idx = *((u16 *)first_skb->data);
+ xenvif_idx_unmap(vif, pending_idx);
+ xenvif_idx_release(vif,
+ pending_idx,
+ XEN_NETIF_RSP_OKAY);
+ }
for (j = start; j < i; j++) {
pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
xenvif_idx_unmap(vif, pending_idx);
@@ -916,6 +970,32 @@ static int xenvif_tx_check_gop(struct xenvif *vif,
err = newerr;
}

+ if (shinfo->frag_list) {
+ first_skb = skb;
+ skb = shinfo->frag_list;
+ shinfo = skb_shinfo(skb);
+ nr_frags = shinfo->nr_frags;
+ start = 0;
+
+ goto check_frags;
+ }
+
+ /* There was a mapping error in the frag_list skb. We have to unmap
+ * the first skb's frags
+ */
+ if (first_skb && err) {
+ int j;
+ shinfo = skb_shinfo(first_skb);
+ pending_idx = *((u16 *)first_skb->data);
+ start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);
+ for (j = start; j < shinfo->nr_frags; j++) {
+ pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
+ xenvif_idx_unmap(vif, pending_idx);
+ xenvif_idx_release(vif, pending_idx,
+ XEN_NETIF_RSP_OKAY);
+ }
+ }
+
*gopp = gop + 1;
return err;
}
@@ -1419,8 +1499,7 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif, int budget)
ret < XEN_NETBK_LEGACY_SLOTS_MAX) ?
PKT_PROT_LEN : txreq.size;

- skb = alloc_skb(data_len + NET_SKB_PAD + NET_IP_ALIGN,
- GFP_ATOMIC | __GFP_NOWARN);
+ skb = xenvif_alloc_skb(data_len);
if (unlikely(skb == NULL)) {
netdev_dbg(vif->dev,
"Can't allocate a skb in start_xmit.\n");
@@ -1428,9 +1507,6 @@ static unsigned xenvif_tx_build_gops(struct xenvif *vif, int budget)
break;
}

- /* Packets passed to netif_rx() must have some headroom. */
- skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
-
if (extras[XEN_NETIF_EXTRA_TYPE_GSO - 1].type) {
struct xen_netif_extra_info *gso;
gso = &extras[XEN_NETIF_EXTRA_TYPE_GSO - 1];
@@ -1492,6 +1568,7 @@ static int xenvif_tx_submit(struct xenvif *vif)
struct xen_netif_tx_request *txp;
u16 pending_idx;
unsigned data_len;
+ struct sk_buff *nskb = NULL;

pending_idx = *((u16 *)skb->data);
txp = &vif->pending_tx_info[pending_idx].req;
@@ -1534,6 +1611,30 @@ static int xenvif_tx_submit(struct xenvif *vif)
pending_idx :
INVALID_PENDING_IDX);

+ if (skb_shinfo(skb)->frag_list) {
+ nskb = skb_shinfo(skb)->frag_list;
+ xenvif_fill_frags(vif, nskb, INVALID_PENDING_IDX);
+ skb->len += nskb->len;
+ skb->data_len += nskb->len;
+ skb->truesize += nskb->truesize;
+ skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+ skb_shinfo(nskb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
+ vif->tx_zerocopy_sent += 2;
+ nskb = skb;
+
+ skb = skb_copy_expand(skb,
+ 0,
+ 0,
+ GFP_ATOMIC | __GFP_NOWARN);
+ if (!skb) {
+ if (net_ratelimit())
+ netdev_dbg(vif->dev,
+ "Can't consolidate skb with too many fragments\n");
+ kfree_skb(nskb);
+ continue;
+ }
+ skb_shinfo(skb)->destructor_arg = NULL;
+ }
if (skb_is_nonlinear(skb) && skb_headlen(skb) < PKT_PROT_LEN) {
int target = min_t(int, skb->len, PKT_PROT_LEN);
__pskb_pull_tail(skb, target - skb_headlen(skb));
@@ -1587,6 +1688,9 @@ static int xenvif_tx_submit(struct xenvif *vif)
}

netif_receive_skb(skb);
+
+ if (nskb)
+ kfree_skb(nskb);
}

return work_done;

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