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Subject[RFC] buddy allocator without bitmap(3) [3/3]
This is the last part.
There is no big change from the previous version.

--Kame
===

This is 4th.
This patch removes bitmap operation from free_pages().

In main lopp of __free_pages_bulk(), we access a "buddy" page.
It is guaranteed that there is no invalid "buddy" in the buddy system, because
all dangerous pages which has possibility to be coalesced with
out-of-range pages are removed in advance by calculate_aligned_end().



---

test-kernel-kamezawa/mm/page_alloc.c | 81 ++++++++++++++++++++++-------------
1 files changed, 51 insertions(+), 30 deletions(-)

diff -puN mm/page_alloc.c~eliminate-bitmap-free mm/page_alloc.c
--- test-kernel/mm/page_alloc.c~eliminate-bitmap-free 2004-09-02 17:03:32.648373272 +0900
+++ test-kernel-kamezawa/mm/page_alloc.c 2004-09-02 17:03:32.653372512 +0900
@@ -157,6 +157,27 @@ static void destroy_compound_page(struct
#endif /* CONFIG_HUGETLB_PAGE */

/*
+ * This function checks whether a page is free && is the buddy
+ * we can do coalesce if
+ * (a) the buddy is free and
+ * (b) the buddy is on the buddy system
+ * (c) the buddy has the same order.
+ * for recording page's order, we use private field and PG_private.
+ *
+ * Because page_count(page) == 0, and zone->lock is aquired.
+ * Atomic page->flags operation is needless here.
+ */
+static inline int page_is_buddy(struct page *page, int order)
+{
+ if (PagePrivate(page) &&
+ (page_order(page) == order) &&
+ !PageReserved(page) &&
+ page_count(page) == 0)
+ return 1;
+ return 0;
+}
+
+/*
* Freeing function for a buddy system allocator.
*
* The concept of a buddy system is to maintain direct-mapped table
@@ -168,9 +189,12 @@ static void destroy_compound_page(struct
* at the bottom level available, and propagating the changes upward
* as necessary, plus some accounting needed to play nicely with other
* parts of the VM system.
- * At each level, we keep one bit for each pair of blocks, which
- * is set to 1 iff only one of the pair is allocated. So when we
- * are allocating or freeing one, we can derive the state of the
+ *
+ * At each level, we keep a list of pages, which are head of chunk of
+ * pages at the level. A page, which is a head of chunks, has its order
+ * in page structure itself and PG_private flag is set. we can get an
+ * order of a page by calling page_order().
+ * So 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
* free, the remainder of the region must be split into blocks.
* If a block is freed, and its buddy is also free, then this
@@ -180,42 +204,41 @@ static void destroy_compound_page(struct
*/

static inline void __free_pages_bulk (struct page *page, struct page *base,
- struct zone *zone, struct free_area *area, unsigned int order)
+ struct zone *zone, unsigned int order)
{
- unsigned long page_idx, index, mask;
-
+ unsigned long page_idx;
+ struct page *coalesced_page;
+ int order_len = 1 << order;
if (order)
destroy_compound_page(page, order);
- mask = (~0UL) << order;
+
page_idx = page - base;
- if (page_idx & ~mask)
+ if (page_idx & (order_len - 1))
BUG();
- index = page_idx >> (1 + order);
+ zone->free_pages += order_len;
+ BUG_ON(bad_range(zone,page));

- zone->free_pages += 1 << order;
while (order < MAX_ORDER-1) {
- struct page *buddy1, *buddy2;
-
- BUG_ON(area >= zone->free_area + MAX_ORDER);
- if (!__test_and_change_bit(index, area->map))
+ struct page *buddy;
+ int buddy_idx;
+ buddy_idx = (page_idx ^ (1 << order));
+ buddy = base + buddy_idx;
+ if (!page_is_buddy(buddy, order))
/*
* the buddy page is still allocated.
*/
break;
-
- /* Move the buddy up one level. */
- buddy1 = base + (page_idx ^ (1 << order));
- buddy2 = base + page_idx;
- BUG_ON(bad_range(zone, buddy1));
- BUG_ON(bad_range(zone, buddy2));
- list_del(&buddy1->lru);
- mask <<= 1;
order++;
- area++;
- index >>= 1;
- page_idx &= mask;
- }
- list_add(&(base + page_idx)->lru, &area->free_list);
+ page_idx &= buddy_idx;
+ list_del(&buddy->lru);
+ /* for propriety of PG_private bit, we clear it */
+ ClearPagePrivate(buddy);
+ }
+ /* record the final order of the page */
+ coalesced_page = base + page_idx;
+ SetPagePrivate(coalesced_page);
+ set_page_order(coalesced_page,order);
+ list_add(&coalesced_page->lru, &zone->free_area[order].free_list);
}

static inline void free_pages_check(const char *function, struct page *page)
@@ -253,12 +276,10 @@ free_pages_bulk(struct zone *zone, int c
struct list_head *list, unsigned int order)
{
unsigned long flags;
- struct free_area *area;
struct page *base, *page = NULL;
int ret = 0;

base = zone->zone_mem_map;
- area = zone->free_area + order;
spin_lock_irqsave(&zone->lock, flags);
zone->all_unreclaimable = 0;
zone->pages_scanned = 0;
@@ -266,7 +287,7 @@ free_pages_bulk(struct zone *zone, int c
page = list_entry(list->prev, struct page, lru);
/* have to delete it as __free_pages_bulk list manipulates */
list_del(&page->lru);
- __free_pages_bulk(page, base, zone, area, order);
+ __free_pages_bulk(page, base, zone, order);
ret++;
}
spin_unlock_irqrestore(&zone->lock, flags);
_

-
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