[NET]: add SO_RCVBUF comment

Put a comment in there explaining why we double the setsockopt()
caller's SO_RCVBUF.  People keep wondering.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 1 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index a96ea7dd0fc1..ed2afdb9ea2d 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -385,7 +385,21 @@ set_sndbuf:
                                 val = sysctl_rmem_max;
 set_rcvbuf:
                         sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
-                        /* FIXME: is this lower bound the right one? */
+                        /*
+                         * We double it on the way in to account for
+                         * "struct sk_buff" etc. overhead.   Applications
+                         * assume that the SO_RCVBUF setting they make will
+                         * allow that much actual data to be received on that
+                         * socket.
+                         *
+                         * Applications are unaware that "struct sk_buff" and
+                         * other overheads allocate from the receive buffer
+                         * during socket buffer allocation.
+                         *
+                         * And after considering the possible alternatives,
+                         * returning the value we actually used in getsockopt
+                         * is the most desirable behavior.
+                         */
                         if ((val * 2) < SOCK_MIN_RCVBUF)
                                 sk->sk_rcvbuf = SOCK_MIN_RCVBUF;
                         else