[PATCH] NOMMU: make SYSV SHM nattch work correctly

Make the SYSV SHM nattch counter work correctly by forcing multiple VMAs to
be produced to represent MAP_SHARED segments, even if they overlap exactly.

Using this test program:

	http://people.redhat.com/~dhowells/doshm.c

Run as:

	doshm sysv

I can see nattch going from one before the patch:

	# /doshm sysv
	Command: sysv
	shmid: 65536
	memory: 0xc3700000
	c0b00000-c0b04000 rw-p 00000000 00:00 0
	c0bb0000-c0bba788 r-xs 00000000 00:0b 14582157  /lib/ld-uClibc-0.9.28.so
	c3180000-c31dede4 r-xs 00000000 00:0b 14582179  /lib/libuClibc-0.9.28.so
	c3520000-c352278c rw-p 00000000 00:0b 13763417  /doshm
	c3584000-c35865e8 r-xs 00000000 00:0b 13763417  /doshm
	c3588000-c358aa00 rw-p 00008000 00:0b 14582157  /lib/ld-uClibc-0.9.28.so
	c3590000-c359b6c0 rw-p 00000000 00:00 0
	c3620000-c3640000 rwxp 00000000 00:00 0
	c3700000-c37fa000 rw-S 00000000 00:06 1411      /SYSV00000000 (deleted)
	c3700000-c37fa000 rw-S 00000000 00:06 1411      /SYSV00000000 (deleted)
	nattch 1

To two after the patch:

	# /doshm sysv
	Command: sysv
	shmid: 0
	memory: 0xc3700000
	c0bb0000-c0bba788 r-xs 00000000 00:0b 14582157  /lib/ld-uClibc-0.9.28.so
	c3180000-c31dede4 r-xs 00000000 00:0b 14582179  /lib/libuClibc-0.9.28.so
	c3320000-c3340000 rwxp 00000000 00:00 0
	c3530000-c35325e8 r-xs 00000000 00:0b 13763417  /doshm
	c3534000-c353678c rw-p 00000000 00:0b 13763417  /doshm
	c3538000-c353aa00 rw-p 00008000 00:0b 14582157  /lib/ld-uClibc-0.9.28.so
	c3590000-c359b6c0 rw-p 00000000 00:00 0
	c35a4000-c35a8000 rw-p 00000000 00:00 0
	c3700000-c37fa000 rw-S 00000000 00:06 1369      /SYSV00000000 (deleted)
	c3700000-c37fa000 rw-S 00000000 00:06 1369      /SYSV00000000 (deleted)
	nattch 2

That's +1 to nattch for each shmat() made.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -826,6 +826,11 @@ unsigned long do_mmap_pgoff(struct file *file,
 		unsigned long pglen = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
 		unsigned long vmpglen;
 
+		/* suppress VMA sharing for shared regions */
+		if (vm_flags & VM_SHARED &&
+		    capabilities & BDI_CAP_MAP_DIRECT)
+			goto dont_share_VMAs;
+
 		for (rb = rb_first(&nommu_vma_tree); rb; rb = rb_next(rb)) {
 			vma = rb_entry(rb, struct vm_area_struct, vm_rb);
 
@@ -859,6 +864,7 @@ unsigned long do_mmap_pgoff(struct file *file,
 			goto shared;
 		}
 
+	dont_share_VMAs:
 		vma = NULL;
 
 		/* obtain the address at which to make a shared mapping