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 <[email protected]>
---
mm/page_alloc.c | 9 ++++++++-
1 files changed, 8 insertions(+), 1 deletion(-)
--- 2.6.16-rc2-git11+/mm/page_alloc.c 2006-02-10 20:03:19.000000000 +0000
+++ 2.6.16-rc2-git11++/mm/page_alloc.c 2006-02-10 20:07:05.000000000 +0000
@@ -56,6 +56,7 @@ long nr_swap_pages;
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:
@@ -173,12 +174,18 @@ static void bad_page(struct page *page)
* 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;
-
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