[rfc] rename page_count for lockless pagecache

[Nick Piggin <npiggin@suse.de>]

In order to force an audit of page_count users (which I have already done
for in-tree users), and to ensure people think about page_count correctly
in future, I propose this (incomplete, RFC) patch to rename page_count.

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -267,13 +267,32 @@ static inline int get_page_unless_zero(s
 	return atomic_inc_not_zero(&page->_count);
 }
 
-static inline int page_count(struct page *page)
+static inline int unstable_page_count(struct page *page)
 {
 	if (unlikely(PageCompound(page)))
 		page = (struct page *)page_private(page);
 	return atomic_read(&page->_count);
 }
 
+/*
+ * Returns true if the page count is zero. The page count is
+ * actually stable if it is zero.
+ */
+static inline int page_count_is_zero(struct page *page)
+{
+	return unstable_page_count(page) == 0;
+}
+
+/*
+ * Returns true if page count of page is less than or equal to c.
+ * PageNoNewRefs must be set on page.
+ */
+static inline int page_count_lessequal(struct page *page, int c)
+{
+	VM_BUG_ON(!PageNoNewRefs(page));
+	return unstable_page_count(page) <= c;
+}
+
 static inline void get_page(struct page *page)
 {
 	if (unlikely(PageCompound(page)))
Index: linux-2.6/mm/hugetlb.c
===================================================================
--- linux-2.6.orig/mm/hugetlb.c
+++ linux-2.6/mm/hugetlb.c
@@ -90,7 +90,7 @@ static struct page *dequeue_huge_page(st
 
 static void free_huge_page(struct page *page)
 {
-	BUG_ON(page_count(page));
+	BUG_ON(!page_count_is_zero(page));
 
 	INIT_LIST_HEAD(&page->lru);
 
@@ -429,7 +429,7 @@ static int hugetlb_cow(struct mm_struct 
 
 	/* If no-one else is actually using this page, avoid the copy
 	 * and just make the page writable */
-	avoidcopy = (page_count(old_page) == 1);
+	avoidcopy = (unstable_page_count(old_page) == 1);
 	if (avoidcopy) {
 		set_huge_ptep_writable(vma, address, ptep);
 		return VM_FAULT_MINOR;
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -1270,7 +1270,7 @@ int vm_insert_page(struct vm_area_struct
 {
 	if (addr < vma->vm_start || addr >= vma->vm_end)
 		return -EFAULT;
-	if (!page_count(page))
+	if (page_count_is_zero(page))
 		return -EINVAL;
 	vma->vm_flags |= VM_INSERTPAGE;
 	return insert_page(vma->vm_mm, addr, page, vma->vm_page_prot);
Index: linux-2.6/mm/migrate.c
===================================================================
--- linux-2.6.orig/mm/migrate.c
+++ linux-2.6/mm/migrate.c
@@ -298,7 +298,7 @@ static int migrate_page_move_mapping(str
 
 	if (!mapping) {
 		/* Anonymous page */
-		if (page_count(page) != 1)
+		if (unstable_page_count(page) != 1)
 			return -EAGAIN;
 		return 0;
 	}
@@ -309,7 +309,7 @@ static int migrate_page_move_mapping(str
 	pslot = radix_tree_lookup_slot(&mapping->page_tree,
  					page_index(page));
 
-	if (page_count(page) != 2 + !!PagePrivate(page) ||
+	if (!page_count_lessequal(page, 2 + !!PagePrivate(page)) ||
 			(struct page *)radix_tree_deref_slot(pslot) != page) {
 		spin_unlock_irq(&mapping->tree_lock);
 		clear_page_nonewrefs(page);
@@ -606,7 +606,7 @@ static int unmap_and_move(new_page_t get
 	if (!newpage)
 		return -ENOMEM;
 
-	if (page_count(page) == 1)
+	if (unstable_page_count(page) == 1)
 		/* page was freed from under us. So we are done. */
 		goto move_newpage;
 
Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c
+++ linux-2.6/mm/page_alloc.c
@@ -192,7 +192,7 @@ static void bad_page(struct page *page)
 		KERN_EMERG "Backtrace:\n",
 		current->comm, page, (int)(2*sizeof(unsigned long)),
 		(unsigned long)page->flags, page->mapping,
-		page_mapcount(page), page_count(page));
+		page_mapcount(page), unstable_page_count(page));
 	dump_stack();
 	page->flags &= ~(1 << PG_lru	|
 			1 << PG_private |
@@ -355,7 +355,7 @@ static inline int page_is_buddy(struct p
 		return 0;
 
 	if (PageBuddy(buddy) && page_order(buddy) == order) {
-		BUG_ON(page_count(buddy) != 0);
+		BUG_ON(!page_count_is_zero(buddy));
 		return 1;
 	}
 	return 0;
@@ -427,7 +427,7 @@ static inline int free_pages_check(struc
 {
 	if (unlikely(page_mapcount(page) |
 		(page->mapping != NULL)  |
-		(page_count(page) != 0)  |
+		(!page_count_is_zero(page)) |
 		(page->flags & (
 			1 << PG_lru	|
 			1 << PG_private |
@@ -576,7 +576,7 @@ static int prep_new_page(struct page *pa
 {
 	if (unlikely(page_mapcount(page) |
 		(page->mapping != NULL)  |
-		(page_count(page) != 0)  |
+		(!page_count_is_zero(page)) |
 		(page->flags & (
 			1 << PG_lru	|
 			1 << PG_private	|
@@ -854,7 +854,7 @@ void split_page(struct page *page, unsig
 	int i;
 
 	VM_BUG_ON(PageCompound(page));
-	VM_BUG_ON(!page_count(page));
+	VM_BUG_ON(page_count_is_zero(page));
 	for (i = 1; i < (1 << order); i++)
 		set_page_refcounted(page + i);
 }
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -586,7 +586,7 @@ void page_remove_rmap(struct page *page,
 			printk (KERN_EMERG "Eeek! page_mapcount(page) went negative! (%d)\n", page_mapcount(page));
 			printk (KERN_EMERG "  page pfn = %lx\n", page_to_pfn(page));
 			printk (KERN_EMERG "  page->flags = %lx\n", page->flags);
-			printk (KERN_EMERG "  page->count = %x\n", page_count(page));
+			printk (KERN_EMERG "  page->count = %x\n", unstable_page_count(page));
 			printk (KERN_EMERG "  page->mapping = %p\n", page->mapping);
 			print_symbol (KERN_EMERG "  vma->vm_ops = %s\n", (unsigned long)vma->vm_ops);
 			if (vma->vm_ops)
Index: linux-2.6/mm/swapfile.c
===================================================================
--- linux-2.6.orig/mm/swapfile.c
+++ linux-2.6/mm/swapfile.c
@@ -73,7 +73,7 @@ void swap_unplug_io_fn(struct backing_de
 		 * condition and it's harmless. However if it triggers without
 		 * swapoff it signals a problem.
 		 */
-		WARN_ON(page_count(page) <= 1);
+		WARN_ON(unstable_page_count(page) <= 1);
 
 		bdi = bdev->bd_inode->i_mapping->backing_dev_info;
 		blk_run_backing_dev(bdi, page);
@@ -355,7 +355,7 @@ int remove_exclusive_swap_page(struct pa
 		return 0;
 	if (PageWriteback(page))
 		return 0;
-	if (page_count(page) != 2) /* 2: us + cache */
+	if (unstable_page_count(page) != 2) /* 2: us + cache */
 		return 0;
 
 	entry.val = page_private(page);
@@ -366,14 +366,14 @@ int remove_exclusive_swap_page(struct pa
 	/* Is the only swap cache user the cache itself? */
 	retval = 0;
 	if (p->swap_map[swp_offset(entry)] == 1) {
-		/* Recheck the page count with the swapcache lock held.. */
-		spin_lock_irq(&swapper_space.tree_lock);
-		if ((page_count(page) == 2) && !PageWriteback(page)) {
+		/* XXX: BUG_ON(PageWriteback(page)) ?? */
+		if ((unstable_page_count(page) == 2) && !PageWriteback(page)) {
+			spin_lock_irq(&swapper_space.tree_lock);
 			__delete_from_swap_cache(page);
+			spin_unlock_irq(&swapper_space.tree_lock);
 			SetPageDirty(page);
 			retval = 1;
 		}
-		spin_unlock_irq(&swapper_space.tree_lock);
 	}
 	spin_unlock(&swap_lock);
 
@@ -412,7 +412,7 @@ void free_swap_and_cache(swp_entry_t ent
 		int one_user;
 
 		BUG_ON(PagePrivate(page));
-		one_user = (page_count(page) == 2);
+		one_user = (unstable_page_count(page) == 2);
 		/* Only cache user (+us), or swap space full? Free it! */
 		/* Also recheck PageSwapCache after page is locked (above) */
 		if (PageSwapCache(page) && !PageWriteback(page) &&
Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -254,7 +254,7 @@ static inline int page_mapping_inuse(str
 
 static inline int is_page_cache_freeable(struct page *page)
 {
-	return page_count(page) - !!PagePrivate(page) == 2;
+	return unstable_page_count(page) - !!PagePrivate(page) == 2;
 }
 
 static int may_write_to_queue(struct backing_dev_info *bdi)

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