[PATCH] compound page: use page[1].lru

If a compound page has its own put_page_testzero destructor (the only current
example is free_huge_page), that is noted in page[1].mapping of the compound
page. But that's rather a poor place to keep it: functions which call
set_page_dirty_lock after get_user_pages (e.g. Infiniband's
__ib_umem_release) ought to be checking first, otherwise set_page_dirty is
liable to crash on what's not the address of a struct address_space.

And now I'm about to make that worse: it turns out that every compound page
needs a destructor, so we can no longer rely on hugetlb pages going their own
special way, to avoid further problems of page->mapping reuse. For example,
not many people know that: on 50% of i386 -Os builds, the first tail page of a
compound page purports to be PageAnon (when its destructor has an odd
address), which surprises page_add_file_rmap.

Keep the compound page destructor in page[1].lru.next instead. And to free up
the common pairing of mapping and index, also move compound page order from
index to lru.prev. Slab reuses page->lru too: but if we ever need slab to use
compound pages, it can easily stack its use above this.

(akpm: decoded version of the above: the tail pages of a compound page now
have ->mapping==NULL, so there's no need for the set_page_dirty[_lock]()
caller to check that they're not compund pages before doing the dirty).

Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/hugetlb.c

···
 	BUG_ON(page_count(page));
 
 	INIT_LIST_HEAD(&page->lru);
-	page[1].mapping = NULL;
+	page[1].lru.next = NULL;			/* reset dtor */
 
 	spin_lock(&hugetlb_lock);
 	enqueue_huge_page(page);
···
 	}
 	spin_unlock(&hugetlb_lock);
 	set_page_count(page, 1);
-	page[1].mapping = (void *)free_huge_page;
+	page[1].lru.next = (void *)free_huge_page;	/* set dtor */
 	for (i = 0; i < (HPAGE_SIZE/PAGE_SIZE); ++i)
 		clear_user_highpage(&page[i], addr);
 	return page;

mm/page_alloc.c

···
  * All pages have PG_compound set.  All pages have their ->private pointing at
  * the head page (even the head page has this).
  *
- * The first tail page's ->mapping, if non-zero, holds the address of the
- * compound page's put_page() function.
- *
- * The order of the allocation is stored in the first tail page's ->index
- * This is only for debug at present.  This usage means that zero-order pages
- * may not be compound.
+ * The first tail page's ->lru.next holds the address of the compound page's
+ * put_page() function.  Its ->lru.prev holds the order of allocation.
+ * This usage means that zero-order pages may not be compound.
  */
 static void prep_compound_page(struct page *page, unsigned long order)
 {
 	int i;
 	int nr_pages = 1 << order;
 
-	page[1].mapping = NULL;
-	page[1].index = order;
+	page[1].lru.next = NULL;			/* set dtor */
+	page[1].lru.prev = (void *)order;
 	for (i = 0; i < nr_pages; i++) {
 		struct page *p = page + i;
 
···
 	int i;
 	int nr_pages = 1 << order;
 
-	if (unlikely(page[1].index != order))
+	if (unlikely((unsigned long)page[1].lru.prev != order))
 		bad_page(page);
 
 	for (i = 0; i < nr_pages; i++) {

mm/swap.c

···
 	if (put_page_testzero(page)) {
 		void (*dtor)(struct page *page);
 
-		dtor = (void (*)(struct page *))page[1].mapping;
+		dtor = (void (*)(struct page *))page[1].lru.next;
 		(*dtor)(page);
 	}
 }