Commit d98c7a09 authored Feb 14, 2006 by Hugh Dickins Committed by Linus Torvalds Feb 14, 2006

[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 <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 41d78ba5

mm/page_alloc.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -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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