Fix errors in tcp_mem[] calculations.

In 2.6.18 a change was made to the tcp_mem[] calculations,
but this causes regressions for some folks up to 2.6.20

The following fix to smooth out the calculation from the
pending 2.6.21 tree by John Heffner fixes the problem for
these folks.

[TCP]: Fix tcp_mem[] initialization.

Change tcp_mem initialization function.  The fraction of total memory
is now a continuous function of memory size, and independent of page
size.
Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

net/ipv4/tcp.c

@@ -2457,11 +2457,18 @@ void __init tcp_init(void)
 		sysctl_max_syn_backlog = 128;
 	}
 
-	/* Allow no more than 3/4 kernel memory (usually less) allocated to TCP */
-	sysctl_tcp_mem[0] = (1536 / sizeof (struct inet_bind_hashbucket)) << order;
-	sysctl_tcp_mem[1] = sysctl_tcp_mem[0] * 4 / 3;
+	/* Set the pressure threshold to be a fraction of global memory that
+	 * is up to 1/2 at 256 MB, decreasing toward zero with the amount of
+	 * memory, with a floor of 128 pages.
+	 */
+	limit = min(nr_all_pages, 1UL<<(28-PAGE_SHIFT)) >> (20-PAGE_SHIFT);
+	limit = (limit * (nr_all_pages >> (20-PAGE_SHIFT))) >> (PAGE_SHIFT-11);
+	limit = max(limit, 128UL);
+	sysctl_tcp_mem[0] = limit / 4 * 3;
+	sysctl_tcp_mem[1] = limit;
 	sysctl_tcp_mem[2] = sysctl_tcp_mem[0] * 2;
 
+	/* Set per-socket limits to no more than 1/128 the pressure threshold */
 	limit = ((unsigned long)sysctl_tcp_mem[1]) << (PAGE_SHIFT - 7);
 	max_share = min(4UL*1024*1024, limit);