net:Add sysctl_max_skb_frags

Devices may have limits on the number of fragments in an skb they support.
Current codebase uses a constant as maximum for number of fragments one
skb can hold and use.
When enabling scatter/gather and running traffic with many small messages
the codebase uses the maximum number of fragments and may thereby violate
the max for certain devices.
The patch introduces a global variable as max number of fragments.

Signed-off-by: Hans Westgaard Ry <hans.westgaard.ry@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

4 files changed, 15 insertions, 2 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 11f935c1a090..4ce9ff7086f4 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -299,6 +299,7 @@ struct sk_buff;
 #else
 #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
 #endif
+extern int sysctl_max_skb_frags;
 
 typedef struct skb_frag_struct skb_frag_t;
 
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b2df375ec9c2..5bf88f58bee7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -79,6 +79,8 @@
 
 struct kmem_cache *skbuff_head_cache __read_mostly;
 static struct kmem_cache *skbuff_fclone_cache __read_mostly;
+int sysctl_max_skb_frags __read_mostly = MAX_SKB_FRAGS;
+EXPORT_SYMBOL(sysctl_max_skb_frags);
 
 /**
  *      skb_panic - private function for out-of-line support
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index 95b6139d710c..a6beb7b6ae55 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -26,6 +26,7 @@ static int zero = 0;
 static int one = 1;
 static int min_sndbuf = SOCK_MIN_SNDBUF;
 static int min_rcvbuf = SOCK_MIN_RCVBUF;
+static int max_skb_frags = MAX_SKB_FRAGS;
 
 static int net_msg_warn;        /* Unused, but still a sysctl */
 
@@ -392,6 +393,15 @@ static struct ctl_table net_core_table[] = {
                 .mode           = 0644,
                 .proc_handler   = proc_dointvec
         },
+        {
+                .procname       = "max_skb_frags",
+                .data           = &sysctl_max_skb_frags,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec_minmax,
+                .extra1         = &one,
+                .extra2         = &max_skb_frags,
+        },
         { }
 };
 
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 19746b3fcbbe..0c36ef4a3f86 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -940,7 +940,7 @@ new_segment:
 
                 i = skb_shinfo(skb)->nr_frags;
                 can_coalesce = skb_can_coalesce(skb, i, page, offset);
-                if (!can_coalesce && i >= MAX_SKB_FRAGS) {
+                if (!can_coalesce && i >= sysctl_max_skb_frags) {
                         tcp_mark_push(tp, skb);
                         goto new_segment;
                 }
@@ -1213,7 +1213,7 @@ new_segment:
 
                         if (!skb_can_coalesce(skb, i, pfrag->page,
                                               pfrag->offset)) {
-                                if (i == MAX_SKB_FRAGS || !sg) {
+                                if (i == sysctl_max_skb_frags || !sg) {
                                         tcp_mark_push(tp, skb);
                                         goto new_segment;
                                 }