up unpinned mmap usable address space from 2 to 3 GB

Make the full 3 GB of process address space available to unpinned mmaps. Previously only the 2 GB from 40000000-c0000000 was available.

The process address space allocation behavior should remain unchanged for non-i386 architectures.

For i386, the new algorithm first allocates from virtual address 40000000 to the start of the stack area (as defined by ulimit -s), then from the start of the text area (08000000) down to 00400000, then from 40000000 down to the sbrk area, then (as a last resort) starts allocating from the unused portion of the stack area.

With this patch, a small java program, simulating a real application, can spin off 3,790 threads. Previously, only 1,750 threads could be spun off.

Due to the fragility of making changes to get_unmapped_area(), this patch is intended 'For Comment Only'.

Against 2.6.7.

Regards,
Joe

================== Test.java

public class Test
{
private static int threadCounter = 0;

  public static synchronized int getThreadNumber() {
    return threadCounter++;
  }

  public static void main(String [] args) {
    for(int i = 0; i < 6000; i++) {
      new Thread(new Runnable() {
          public void run() {
            System.err.println("Thread " + getThreadNumber());
            while(true) { 
              try {
                Thread.currentThread().sleep(10000);
              } catch(InterruptedException ie) {
                // Don't care
              }
            }
          }
        }
      ).start();
    }

}
}

================== compilation instructions

Download Java 2 SDK 1.4.2_04 linux i586 from java.sun.com, install javac Test.java
java -Xmx1000M -Xms1000M -cp . Test

================== patch

diff -ura base/include/asm-i386/processor.h new/include/asm-i386/processor.h
@@ -301,6 +301,9 @@
  * space during mmap's.
  */
 #define TASK_UNMAPPED_BASE	(PAGE_ALIGN(TASK_SIZE / 3))
+#define TASK_UNMAPPED_BASE_2	(PAGE_ALIGN(16 * PAGE_SIZE))
+#define TASK_SLOP_FUNCTION	(current->rlim[RLIMIT_STACK].rlim_cur)
+#define MIN_TASK_SLOP		(64 * PAGE_SIZE)
 
 /*
  * Size of io_bitmap, covering ports 0 to 0x3ff.