: fix the bug of __free_pages() of mm/page_alloc.c

[PATCH]: fix the bug of __free_pages() of mm/page_alloc.c
=========================================================

The buddy allocator's __free_pages() function seems to be buggy.

The following codes are from kernel 2.6.10:

fastcall void __free_pages(struct page *page, unsigned int order)

{
	if (!PageReserved(page) && put_page_testzero(page)) {
		if (order == 0)
			free_hot_page(page);
		else
			__free_pages_ok(page, order);
	}
}

As you know, before truely freeing all pages, this function calls put_page_testzero(page) to
drop the refcount of the pages.

But, in fact the macro put_page_testzero(page) **only** drops **one** page's refcount.
Therefore, if (order > 0), the refcounts of (page+1) .. (page+(1<<order)-1) are unchanged!
This will cause __free_pages_ok() to dump stack, because it finds some pages' page_count()
are not zero!

The following is the OOPS of this bug (I make kernel print verbose info):

slab cache: size-32768 is to be destroyed! __free_pages: order=3, we are freeing page: 001d8740-001d8858 page 001d8740 count: 1
page 001d8768 count: 1
page 001d8790 count: 1
page 001d87b8 count: 1
page 001d87e0 count: 1
page 001d8808 count: 1
page 001d8830 count: 1
page 001d8858 count: 1
put_page_testzero() has been called!
page 001d8740 count: 0
page 001d8768 count: 1
page 001d8790 count: 1
page 001d87b8 count: 1
page 001d87e0 count: 1
page 001d8808 count: 1
page 001d8830 count: 1
page 001d8858 count: 1
Bad page state at __free_pages_ok (in process 'hello', mem_map 00139000, order 3, page 001d8768, i
1)
flags:0x20000000 mapping:00000000 mapped:0 count:1

Backtrace:
Call Trace:
 [<00034b9c>] __free_pages_ok+0x12c/0x1f0
 [<000358d0>] __free_pages+0xd0/0x1a0
 [<000359f4>] free_pages+0x54/0x70
 [<00038df4>] slab_destroy+0x114/0x1e0
 [<0003a0b4>] reap_timer_fnc+0x194/0x300
 [<0001e058>] do_timer+0x58/0x600
 [<0001de7c>] run_timer_softirq+0x10c/0x200
 [<0001ace4>] do_softirq+0xb4/0x140
 [<00124548>] init_bio+0x8/0x200
 [<0000be68>] do_IRQ+0x208/0x280
 [<00124548>] init_bio+0x8/0x200
 [<000e22f8>] handle_IRQ+0x58/0x60
 [<000d7344>] blkdev_ioctl+0x304/0x8f0
 [<00122ca0>] free_bootmem_core+0xa0/0xf0
 [<00124548>] init_bio+0x8/0x200
 [<0001101c>] try_to_wake_up+0x14c/0x250
 [<00011140>] wake_up_process+0x20/0x30
 [<000370a0>] pdflush_operation+0xc0/0xe0
 [<00036ab4>] wb_timer_fn+0x24/0x70
 [<0001de7c>] run_timer_softirq+0x10c/0x200
 [<00123a88>] free_area_init_node+0x158/0x450
 [<00090bb8>] group_reserve_blocks+0x8/0x90
 [<0000ee00>] do_gettimeofday+0xb0/0x160
 ......

Please confirm me! This is a patch to fix this bug:
---------------------------------------------------

@@ -786,9 +786,19 @@
 		free_hot_cold_page(pvec->pages[i], pvec->cold);
 }
 
+static inline int drop_pages_testzero(struct page *page, unsigned int order)
+{
+	int i, ret = 1;
+	struct page *pg = page;
+	
+	for (i = 0; i < (1 << order); i++, pg++)
+		ret &= put_page_testzero(pg);
+	return ret;
+}
+
 fastcall void __free_pages(struct page *page, unsigned int order)
 {
-	if (!PageReserved(page) && put_page_testzero(page)) {
+	if (!PageReserved(page) && drop_pages_testzero(page, order)) {
 		if (order == 0)
 			free_hot_page(page);
 		else

Rongkai Zhan

When you allocate a page with order > 0, the first 0-order page has a refcount of 1, and the remaining 0-order pages have a refcount of 0.

If you're triggering this check, I suspect you're fiddling about with the individual pages (using get_page on them individually?) which is a no-no.

Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core

Thank you for telling me this point.

Oh, I forget to tell you that my CPU has no MMU, sorry:-) Let's see the function set_page_refs() which is called by prep_new_page() function:

static inline void set_page_refs(struct page *page, int order) {

#ifdef CONFIG_MMU
	set_page_count(page, 1);
#else
	int i;

	/*
	 * We need to reference all the pages for this order, otherwise if
	 * anyone accesses one of the pages with (get/put) it will be freed.
	 */
	for (i = 0; i < (1 << order); i++)
		set_page_count(page+i, 1);

#endif /* CONFIG_MMU */
}

We can see that it sets all pages' refcount to 1 when there is no MMU.

My previous patch is wrong. Here is new one:
--------------------------------------------

@@ -787,8 +787,23 @@
 }
 
 fastcall void __free_pages(struct page *page, unsigned int order)
-{
-	if (!PageReserved(page) && put_page_testzero(page)) {
+{	
+	if (!PageReserved(page)) {
+#ifdef CONFIG_MMU
+		if (!put_page_testzero(page))
+			return;
+#else
+		int i, result = 1;
+
+		/*
+		 * We need to de-reference all the pages for this order -- see

set_page_refs()
+ */
+ for (i = 0; i < (1 << order); i++)
+ result &= put_page_testzero(page);
+ if (!result)
+ BUG();
+#endif /* CONFIG_MMU */
+
if (order == 0)
free_hot_page(page);
else

Rongkai Zhan

@@ -788,7 +788,22 @@
 
 fastcall void __free_pages(struct page *page, unsigned int order)
 {
-	if (!PageReserved(page) && put_page_testzero(page)) {
+	if (!PageReserved(page)) {
+#ifdef CONFIG_MMU
+		if (!put_page_testzero(page))
+			return;
+#else
+		int i, result = 1;
+
+		/*
+		 * We need to de-reference all the pages for this order -- see

set_page_refs()
+ */
+ for (i = 0; i < (1 << order); i++)
+ result &= put_page_testzero(page+i);
+ if (!result)
+ BUG();
+#endif /* CONFIG_MMU */
+
if (order == 0)
free_hot_page(page);
else

Rongkai Zhan

@@ -788,7 +788,22 @@
 
 fastcall void __free_pages(struct page *page, unsigned int order)
 {
-	if (!PageReserved(page) && put_page_testzero(page)) {
+	if (!PageReserved(page)) {
+#ifdef CONFIG_MMU
+		if (!put_page_testzero(page))
+			return;
+#else
+		int i, result = 1;
+
+		/*
+		 * We need to de-reference all the pages for this order -- see

set_page_refs()
+ */
+ for (i = 0; i < (1 << order); i++)
+ result &= put_page_testzero(page+i);
+ if (!result)
+ BUG();
+#endif /* CONFIG_MMU */
+
if (order == 0)
free_hot_page(page);
else

Rongkai Zhan