Check for PageSlab in arch flush_dcache_page to avoid triggering VM_BUG_ON

We added a VM_BUG_ON in include/linux/mm.h to detect page cache
functions used on slab pages. However, arches that have virtual
caches may call flush_dcache_page for slab pages since they may
have been put on scatter gather lists.

Modify the functions in the affected arches to check for PageSlab()
and use a NULL mapping if such a page is encountered. This may
only be necessary for parisc and arm since sparc64 and xtensa do not
scan over processes mapping a page but I have modified those two
arches also for correctnesses sake since they use page_mapping() in
flush_dcache_page().

If we have done that then we no longer need to return a NULL mapping
in page_mapping() for slab pages (Its a bit strange to have that given
the earlier VM_BUG_ON(PageSlab(page)).

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6/arch/arm/mm/flush.c
===================================================================
--- linux-2.6.orig/arch/arm/mm/flush.c	2007-01-10 19:33:45.000000000 -0800
+++ linux-2.6/arch/arm/mm/flush.c	2007-07-23 13:02:56.000000000 -0700
@@ -188,7 +188,17 @@ static void __flush_dcache_aliases(struc
  */
 void flush_dcache_page(struct page *page)
 {
-	struct address_space *mapping = page_mapping(page);
+	struct address_space *mapping;
+
+	/*
+	 * This function is special in that a page struct obtained via
+	 * virt_to_page from a slab object may be passed to it. However, slab
+	 * allocators may use the mapping field for their own purposes. As a
+	 * result mapping may be != NULL here although the page is not mapped.
+	 * Slab objects are never mapped into user space so use NULL for that
+	 * special case.
+	 */
+	mapping = PageSlab(page) ? NULL : page_mapping(page);
 
 #ifndef CONFIG_SMP
 	if (mapping && !mapping_mapped(mapping))
Index: linux-2.6/arch/parisc/kernel/cache.c
===================================================================
--- linux-2.6.orig/arch/parisc/kernel/cache.c	2007-05-22 11:42:42.000000000 -0700
+++ linux-2.6/arch/parisc/kernel/cache.c	2007-07-23 13:02:56.000000000 -0700
@@ -339,7 +339,7 @@ __flush_cache_page(struct vm_area_struct
 
 void flush_dcache_page(struct page *page)
 {
-	struct address_space *mapping = page_mapping(page);
+	struct address_space *mapping;
 	struct vm_area_struct *mpnt;
 	struct prio_tree_iter iter;
 	unsigned long offset;
@@ -347,6 +347,15 @@ void flush_dcache_page(struct page *page
 	pgoff_t pgoff;
 	unsigned long pfn = page_to_pfn(page);
 
+	/*
+	 * This function is special in that a page struct obtained via
+	 * virt_to_page from a slab object may be passed to it. However, slab
+	 * allocators may use the mapping field for their own purposes. As a
+	 * result mapping may be != NULL here although the page is not mapped.
+	 * Slab objects are never mapped into user space so use NULL for that
+	 * special case.
+	 */
+	mapping = PageSlab(page) ? NULL : page_mapping(page);
 
 	if (mapping && !mapping_mapped(mapping)) {
 		set_bit(PG_dcache_dirty, &page->flags);
Index: linux-2.6/arch/sparc64/mm/init.c
===================================================================
--- linux-2.6.orig/arch/sparc64/mm/init.c	2007-05-30 11:19:44.000000000 -0700
+++ linux-2.6/arch/sparc64/mm/init.c	2007-07-23 13:02:56.000000000 -0700
@@ -339,7 +339,15 @@ void flush_dcache_page(struct page *page
 
 	this_cpu = get_cpu();
 
-	mapping = page_mapping(page);
+	/*
+	 * This function is special in that a page struct obtained via
+	 * virt_to_page from a slab object may be passed to it. However, slab
+	 * allocators may use the mapping field for their own purposes. As a
+	 * result mapping may be != NULL here although the page is not mapped.
+	 * Slab objects are never mapped into user space so use NULL for that
+	 * special case.
+	 */
+	mapping = PageSlab(page) ? NULL : page_mapping(page);
 	if (mapping && !mapping_mapped(mapping)) {
 		int dirty = test_bit(PG_dcache_dirty, &page->flags);
 		if (dirty) {
Index: linux-2.6/arch/xtensa/mm/init.c
===================================================================
--- linux-2.6.orig/arch/xtensa/mm/init.c	2007-06-07 19:25:13.000000000 -0700
+++ linux-2.6/arch/xtensa/mm/init.c	2007-07-23 13:02:56.000000000 -0700
@@ -433,7 +433,7 @@ void copy_user_page(void* to, void* from
 void flush_dcache_page(struct page *page)
 {
 	unsigned long addr = __pa(page_address(page));
-	struct address_space *mapping = page_mapping(page);
+	struct address_space *mapping;
 
 	__flush_invalidate_dcache_page_phys(addr);
 
@@ -442,6 +442,15 @@ void flush_dcache_page(struct page *page
 
 	/* If this page hasn't been mapped, yet, handle I$/D$ coherency later.*/
 #if 0
+	/*
+	 * This function is special in that a page struct obtained via
+	 * virt_to_page from a slab object may be passed to it. However, slab
+	 * allocators may use the mapping field for their own purposes. As a
+	 * result mapping may be != NULL although the page is not mapped.
+	 * Slab objects are never mapped into user space so use NULL for that
+	 * special case.
+	 */
+	mapping = PageSlab(page) ? NULL : page_mapping(page);
 	if (mapping && !mapping_mapped(mapping))
 		clear_bit(PG_cache_clean, &page->flags);
 	else
Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h	2007-07-23 13:02:16.000000000 -0700
+++ linux-2.6/include/linux/mm.h	2007-07-23 13:03:22.000000000 -0700
@@ -630,10 +630,6 @@ static inline struct address_space *page
 	VM_BUG_ON(PageSlab(page));
 	if (unlikely(PageSwapCache(page)))
 		mapping = &swapper_space;
-#ifdef CONFIG_SLUB
-	else if (unlikely(PageSlab(page)))
-		mapping = NULL;
-#endif
 	else if (unlikely((unsigned long)mapping & PAGE_MAPPING_ANON))
 		mapping = NULL;
 	return mapping;