Commit d98c7a09 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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[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: default avatarHugh Dickins <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 41d78ba5
......@@ -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] = NULL; /* set dtor */
page[1] = (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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