[PATCH] rmap: additional diagnostics in page_remove_rmap()

We seem to be hitting this assertion failure too often for it to be
hardware bugs.

Cc: Hugh Dickins <hugh@veritas.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/rmap.c

diff --git a/mm/rmap.c b/mm/rmap.c
index 6f3f7db..66ec430 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -514,6 +514,13 @@
 void page_remove_rmap(struct page *page)
 {
 	if (atomic_add_negative(-1, &page->_mapcount)) {
+		if (page_mapcount(page) < 0) {
+			printk (KERN_EMERG "Eeek! page_mapcount(page) went negative! (%d)\n", page_mapcount(page));
+			printk (KERN_EMERG "  page->flags = %lx\n", page->flags);
+			printk (KERN_EMERG "  page->count = %x\n", page_count(page));
+			printk (KERN_EMERG "  page->mapping = %p\n", page->mapping);
+		}
+
 		BUG_ON(page_mapcount(page) < 0);
 		/*
 		 * It would be tidy to reset the PageAnon mapping here,