[PATCH] nr_pagecache can go negative

We use per-cpu counters for the system-wide pagecache accounting.  The
counters spill into the global nr_pagecache atomic_t when they underflow or
overflow.

Hence it is possible, under weird circumstances, for nr_pagecache to go
negative.  Anton says he has hit this.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/pagemap.h

@@ -8,6 +8,7 @@
 #include <linux/fs.h>
 #include <linux/list.h>
 #include <linux/highmem.h>
+#include <linux/compiler.h>
 #include <asm/uaccess.h>
 #include <linux/gfp.h>
 
@@ -136,7 +137,10 @@ static inline void pagecache_acct(int count)
 
 static inline unsigned long get_page_cache_size(void)
 {
-        return atomic_read(&nr_pagecache);
+	int ret = atomic_read(&nr_pagecache);
+	if (unlikely(ret < 0))
+		ret = 0;
+	return ret;
 }
 
 static inline pgoff_t linear_page_index(struct vm_area_struct *vma,