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[PATCH] Fix buddy list race that could lead to page lru list corruptions

Browse source 
Rohit found an obscure bug causing buddy list corruption.

page_is_buddy is using a non-atomic test (PagePrivate && page_count == 0)
to determine whether or not a free page's buddy is itself free and in the
buddy lists.

Each of the conjuncts may be true at different times due to unrelated
conditions, so the non-atomic page_is_buddy test may find each conjunct to
be true even if they were not both true at the same time (ie. the page was
not on the buddy lists).

Signed-off-by: Martin Bligh <mbligh@google.com>
Signed-off-by: Rohit Seth <rohitseth@google.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Nick Piggin 2006-04-10 11:21:48 +10:00 committed by Linus Torvalds
parent c3a9d6541f
commit 676165a8af
3 changed files with 27 additions and 17 deletions
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5
include/linux/mm.h
View file

@ -229,10 +229,9 @@ struct page {
unsigned long private; /* Mapping-private opaque data:
* usually used for buffer_heads
* if PagePrivate set; used for
* swp_entry_t if PageSwapCache.
* When page is free, this
* swp_entry_t if PageSwapCache;
* indicates order in the buddy
* system.
* system if PG_buddy is set.
*/
struct address_space *mapping; /* If low bit clear, points to
* inode address_space, or NULL.

8
include/linux/page-flags.h
View file

@ -74,7 +74,9 @@
#define PG_mappedtodisk 16 /* Has blocks allocated on-disk */
#define PG_reclaim 17 /* To be reclaimed asap */
#define PG_nosave_free 18 /* Free, should not be written */
#define PG_uncached 19 /* Page has been mapped as uncached */
#define PG_buddy 19 /* Page is free, on buddy lists */
#define PG_uncached 20 /* Page has been mapped as uncached */
/*
* Global page accounting. One instance per CPU. Only unsigned longs are
@ -317,6 +319,10 @@ extern void __mod_page_state_offset(unsigned long offset, unsigned long delta);
#define SetPageNosaveFree(page) set_bit(PG_nosave_free, &(page)->flags)
#define ClearPageNosaveFree(page) clear_bit(PG_nosave_free, &(page)->flags)
#define PageBuddy(page) test_bit(PG_buddy, &(page)->flags)
#define __SetPageBuddy(page) __set_bit(PG_buddy, &(page)->flags)
#define __ClearPageBuddy(page) __clear_bit(PG_buddy, &(page)->flags)
#define PageMappedToDisk(page) test_bit(PG_mappedtodisk, &(page)->flags)
#define SetPageMappedToDisk(page) set_bit(PG_mappedtodisk, &(page)->flags)
#define ClearPageMappedToDisk(page) clear_bit(PG_mappedtodisk, &(page)->flags)

31
mm/page_alloc.c
View file

@ -151,7 +151,8 @@ static void bad_page(struct page *page)
1 << PG_reclaim |
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback );
1 << PG_writeback |
1 << PG_buddy );
set_page_count(page, 0);
reset_page_mapcount(page);
page->mapping = NULL;
@ -236,12 +237,12 @@ static inline unsigned long page_order(struct page *page) {
static inline void set_page_order(struct page *page, int order) {
set_page_private(page, order);
__SetPagePrivate(page);
__SetPageBuddy(page);
}
static inline void rmv_page_order(struct page *page)
{
__ClearPagePrivate(page);
__ClearPageBuddy(page);
set_page_private(page, 0);
}
@ -280,11 +281,13 @@ __find_combined_index(unsigned long page_idx, unsigned int order)
* This function checks whether a page is free && is the buddy
* we can do coalesce a page and its buddy if
* (a) the buddy is not in a hole &&
* (b) the buddy is free &&
* (c) the buddy is on the buddy system &&
* (d) a page and its buddy have the same order.
* for recording page's order, we use page_private(page) and PG_private.
* (b) the buddy is in the buddy system &&
* (c) a page and its buddy have the same order.
*
* For recording whether a page is in the buddy system, we use PG_buddy.
* Setting, clearing, and testing PG_buddy is serialized by zone->lock.
*
* For recording page's order, we use page_private(page).
*/
static inline int page_is_buddy(struct page *page, int order)
{
@ -293,10 +296,10 @@ static inline int page_is_buddy(struct page *page, int order)
return 0;
#endif
if (PagePrivate(page) &&
(page_order(page) == order) &&
page_count(page) == 0)
if (PageBuddy(page) && page_order(page) == order) {
BUG_ON(page_count(page) != 0);
return 1;
}
return 0;
}
@ -313,7 +316,7 @@ static inline int page_is_buddy(struct page *page, int order)
* as necessary, plus some accounting needed to play nicely with other
* parts of the VM system.
* At each level, we keep a list of pages, which are heads of continuous
* free pages of length of (1 << order) and marked with PG_Private.Page's
* free pages of length of (1 << order) and marked with PG_buddy. Page's
* order is recorded in page_private(page) field.
* So when we are allocating or freeing one, we can derive the state of the
* other. That is, if we allocate a small block, and both were
@ -376,7 +379,8 @@ static inline int free_pages_check(struct page *page)
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |
1 << PG_reserved ))))
1 << PG_reserved |
1 << PG_buddy ))))
bad_page(page);
if (PageDirty(page))
__ClearPageDirty(page);
@ -524,7 +528,8 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |
1 << PG_reserved ))))
1 << PG_reserved |
1 << PG_buddy ))))
bad_page(page);
/*