[RFC] buddy allocator without bitmap(3) [1/3]

From: Hiroyuki KAMEZAWA
Date: Thu Sep 02 2004 - 02:59:04 EST

This part is for initialization.

-- Kame

======

This patch is 2nd.
This implements some initialization for the buddy allocator.

New function: calculate_aligned_end()

calculate_aligned_end() removes some pages from system for removing invalid
mem_map access from __free_pages_bulk() main loop.(This is in 4th patch)

See below
================== main loop in __free_pages_bulk(page,order) ===========
while (order < MAX_ORDER) {
struct page *buddy;
......
buddy_idx = page_idx ^ (1 << order);
buddy = zone->zone_mem_map + buddy_idx;
if (page_count(buddy) !=0 --------------------(*)
.......
}
===============================================================
At (*), we have to guarantee that "buddy" is a valid page struct
in a valid zone.

Let MASK = (1 << (MAX_ORDER - 1)) - 1.
A page of index 'X' can be coalesced with pages from (X &~MASK) to (X | mask).
If both of a start index and length of a mem_map are multiples
of (1 << (MAX_ORDER - 1)), all pages have its valid buddies in all order.
Namely, if a mem_map is aligned with (1 << (MAX_ORDER -1)),
there is no invalid "buddy" access in (*).

If a mem_map is unaligned, there will be invalid access in (*).
For fixing this, it looks good to force the mem_map to be aligned.

(example) IA32's MAX_ORDER is 11. The alignment of buddy system is
(1 << (11 - 1))=1024. To make a mem_map aligned is not so difficult.

But , for example, IA64 kernel's MAX_ORDER is 18 !!!
Forcing a mem_map to be aligned is not good in general.
So, I uses another approach.

calculate_aligned_end() discards some pages which is to be coalesced with
invalid "buddy". It is called from memmap_init() which is called per
contiguous mem_map.
It removes some pages from the mem_map to guarantee that there is no
invalid "buddy" access in (*).

What is done in calculate_aligned_end() ? :
(1) It checks whether a mem_map is aligned or not.
(2) If the start of mem_map is unaligned, it find pages which have
possibility of being coalesced with invalid pages.
It marks them with PG_locked.
(3) If the end of mem_map is unaligned, it marks the end page with PG_locked.

Pages marked with PG_locked is removed by free_pages_at_init() and
is not added to the buddy system.

I think many machines' mem_map has good alignment and there are usually no
removed pages :).

-- Kame


---

include/linux/gfp.h | 0
test-kernel-kamezawa/mm/bootmem.c | 9 --
test-kernel-kamezawa/mm/page_alloc.c | 140 +++++++++++++++++++++++++----------
3 files changed, 103 insertions(+), 46 deletions(-)

diff -puN mm/page_alloc.c~eliminate-bitmap-init mm/page_alloc.c
--- test-kernel/mm/page_alloc.c~eliminate-bitmap-init 2004-09-02 15:18:48.572933656 +0900
+++ test-kernel-kamezawa/mm/page_alloc.c 2004-09-02 15:45:14.692806784 +0900
@@ -917,6 +917,33 @@ fastcall void free_pages(unsigned long a
EXPORT_SYMBOL(free_pages);

/*
+ * This function doesn't pass PG_locked pages to buddy allocator.
+ * PG_locked pages are silently removed from buddy system, which are
+ * marked with PG_private and set page_order() to INVALID_PAGE_ORDER.
+ */
+int __init free_pages_at_init(struct page *base, unsigned int order)
+{
+ struct page *page;
+ int nr_pages = (1 << order);
+ int nr_freed_pages;
+
+ if (PageReserved(base) || !put_page_testzero(base))
+ return 0;
+
+ for( page = base,nr_freed_pages = 0; page != base + nr_pages; page++) {
+ if (PageLocked(page)) {
+ /* this page is out of buddy system */
+ SetPagePrivate(page);
+ set_page_order(page, PAGE_INVALID_ORDER);
+ } else {
+ free_hot_page(page);
+ nr_freed_pages++;
+ }
+ }
+ return nr_freed_pages;
+}
+
+/*
* Total amount of free (allocatable) RAM:
*/
unsigned int nr_free_pages(void)
@@ -1499,6 +1526,67 @@ static void __init calculate_zone_totalp
printk(KERN_DEBUG "On node %d totalpages: %lu\n", pgdat->node_id, realtotalpages);
}

+/*
+ * (1) We check whether the mem_map is aligned to MAX_ORDER or not.
+ * (2) If the mem_map is not aligned in its start address, we find pages
+ * which are top of buddy-list in each order and check whether
+ * their buddy are out of mem_map.If so, we mark them with PG_locked.
+ * (3) If the mem_map is not aligned in its end addres,we mark the end page
+ * of it with PG_locked.
+ *
+ * Marked pages are not added to buddy system. Please see free_pages_at_init().
+ */
+
+static void __init calculate_aligned_end(struct zone *zone,
+ unsigned long start_pfn,
+ int nr_pages)
+{
+ struct page *base;
+ unsigned long alignment_mask;
+ long start_idx, end_idx;
+
+ start_idx = start_pfn - zone->zone_start_pfn;
+ end_idx = start_idx + nr_pages - 1;
+ alignment_mask = (1 << MAX_ORDER) - 1;
+ base = zone->zone_mem_map;
+ printk("calculate_aligned_end() %lx %lx\n",start_pfn, nr_pages);
+
+ if (start_idx & alignment_mask) {
+ long edge_idx, buddy_idx;
+ int order;
+ for (edge_idx = start_idx,order = 0;
+ order < MAX_ORDER;
+ order++) {
+ if (edge_idx > end_idx)
+ break;
+ buddy_idx = edge_idx ^ (1 << order);
+ if (buddy_idx > end_idx)
+ break;
+ if (buddy_idx < edge_idx) {
+ /*
+ * Reserve the top page in this order
+ * as the stopper for buddy allocator.
+ * Because this page is an only page
+ * which has an out of range buddy in
+ * this order.
+ */
+ printk("victim top page %lx\n",
+ zone->zone_start_pfn + edge_idx);
+ SetPageLocked(base + edge_idx);
+ edge_idx += (1 << order);
+ }
+ }
+ }
+ if ((end_idx & alignment_mask) != alignment_mask) {
+ /*
+ * Reserve the last page as the stopper for buddy allocator.
+ * Any coalescing including this page will not occur.
+ */
+ printk("victim end page %lx\n",zone->zone_start_pfn + end_idx);
+ SetPageLocked(base + end_idx);
+ }
+ return;
+}

/*
* Initially all pages are reserved - free ones are freed
@@ -1510,9 +1598,13 @@ void __init memmap_init_zone(unsigned lo
{
struct page *start = pfn_to_page(start_pfn);
struct page *page;
-
+ unsigned long saved_start_pfn = start_pfn;
+ int zoneid = NODEZONE(nid, zone);
+ struct zone *zonep;
+ zonep = zone_table[zoneid];
+
for (page = start; page < (start + size); page++) {
- set_page_zone(page, NODEZONE(nid, zone));
+ set_page_zone(page, zoneid);
set_page_count(page, 0);
reset_page_mapcount(page);
SetPageReserved(page);
@@ -1524,51 +1616,19 @@ void __init memmap_init_zone(unsigned lo
#endif
start_pfn++;
}
-}
-
-/*
- * Page buddy system uses "index >> (i+1)", where "index" is
- * at most "size-1".
- *
- * The extra "+3" is to round down to byte size (8 bits per byte
- * assumption). Thus we get "(size-1) >> (i+4)" as the last byte
- * we can access.
- *
- * The "+1" is because we want to round the byte allocation up
- * rather than down. So we should have had a "+7" before we shifted
- * down by three. Also, we have to add one as we actually _use_ the
- * last bit (it's [0,n] inclusive, not [0,n[).
- *
- * So we actually had +7+1 before we shift down by 3. But
- * (n+8) >> 3 == (n >> 3) + 1 (modulo overflows, which we do not have).
- *
- * Finally, we LONG_ALIGN because all bitmap operations are on longs.
- */
-unsigned long pages_to_bitmap_size(unsigned long order, unsigned long nr_pages)
-{
- unsigned long bitmap_size;
-
- bitmap_size = (nr_pages-1) >> (order+4);
- bitmap_size = LONG_ALIGN(bitmap_size+1);
+ /*
+ * calling calculate_aligned_end(zone) is need to be called per
+ * a contiguous mem_map.
+ */
+ calculate_aligned_end(zonep, saved_start_pfn, size);

- return bitmap_size;
}

void zone_init_free_lists(struct pglist_data *pgdat, struct zone *zone, unsigned long size)
{
int order;
- for (order = 0; ; order++) {
- unsigned long bitmap_size;
-
+ for (order = 0 ; order < MAX_ORDER ; order++) {
INIT_LIST_HEAD(&zone->free_area[order].free_list);
- if (order == MAX_ORDER-1) {
- zone->free_area[order].map = NULL;
- break;
- }
-
- bitmap_size = pages_to_bitmap_size(order, size);
- zone->free_area[order].map =
- (unsigned long *) alloc_bootmem_node(pgdat, bitmap_size);
}
}

diff -puN include/linux/gfp.h~eliminate-bitmap-init include/linux/gfp.h
diff -puN mm/bootmem.c~eliminate-bitmap-init mm/bootmem.c
--- test-kernel/mm/bootmem.c~eliminate-bitmap-init 2004-09-02 15:18:48.576933048 +0900
+++ test-kernel-kamezawa/mm/bootmem.c 2004-09-02 15:18:48.583931984 +0900
@@ -277,7 +277,6 @@ static unsigned long __init free_all_boo
if (gofast && v == ~0UL) {
int j;

- count += BITS_PER_LONG;
__ClearPageReserved(page);
set_page_count(page, 1);
for (j = 1; j < BITS_PER_LONG; j++) {
@@ -285,17 +284,16 @@ static unsigned long __init free_all_boo
prefetchw(page + j + 16);
__ClearPageReserved(page + j);
}
- __free_pages(page, ffs(BITS_PER_LONG)-1);
+ count += free_pages_at_init(page,ffs(BITS_PER_LONG)-1);
i += BITS_PER_LONG;
page += BITS_PER_LONG;
} else if (v) {
unsigned long m;
for (m = 1; m && i < idx; m<<=1, page++, i++) {
if (v & m) {
- count++;
__ClearPageReserved(page);
set_page_count(page, 1);
- __free_page(page);
+ count += free_pages_at_init(page,0);
}
}
} else {
@@ -312,10 +310,9 @@ static unsigned long __init free_all_boo
page = virt_to_page(bdata->node_bootmem_map);
count = 0;
for (i = 0; i < ((bdata->node_low_pfn-(bdata->node_boot_start >> PAGE_SHIFT))/8 + PAGE_SIZE-1)/PAGE_SIZE; i++,page++) {
- count++;
__ClearPageReserved(page);
set_page_count(page, 1);
- __free_page(page);
+ count += free_pages_at_init(page,0);
}
total += count;
bdata->node_bootmem_map = NULL;

_




--
--the clue is these footmarks leading to the door.--
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>


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