[PATCH] compound page: default destructor

Somehow I imagined that calling a NULL destructor would free a compound page
rather than oopsing. No, we must supply a default destructor, __free_pages_ok
using the order noted by prep_compound_page. hugetlb can still replace this
as before with its own free_huge_page pointer.

The case that needs this is not common: rarely does put_compound_page's
put_page_testzero bring the count down to 0. But if get_user_pages is applied
to some part of a compound page, without immediate release (e.g. AIO or
Infiniband), then it's possible for its put_page to come after the containing
vma has been unmapped and the driver done its free_pages.

That's just the kind of case compound pages are supposed to be guarding
against (but Nick points out, nor did PageReserved handle this right).

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

mm/page_alloc.c

···
 int percpu_pagelist_fraction;
 
 static void fastcall free_hot_cold_page(struct page *page, int cold);
+static void __free_pages_ok(struct page *page, unsigned int order);
 
 /*
  * results with 256, 32 in the lowmem_reserve sysctl:
···
  * put_page() function.  Its ->lru.prev holds the order of allocation.
  * This usage means that zero-order pages may not be compound.
  */
+
+static void free_compound_page(struct page *page)
+{
+	__free_pages_ok(page, (unsigned long)page[1].lru.prev);
+}
+
 static void prep_compound_page(struct page *page, unsigned long order)
 {
 	int i;
 	int nr_pages = 1 << order;
 
-	page[1].lru.next = NULL;			/* set dtor */
+	page[1].lru.next = (void *)free_compound_page;	/* set dtor */
 	page[1].lru.prev = (void *)order;
 	for (i = 0; i < nr_pages; i++) {
 		struct page *p = page + i;