The ftruncate() in this program:

#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

int clean = 64 * 1024 * 1024;
int dirty = 64 * 1024 * 1024;

main()
{
	int fd = open("foo", O_RDWR|O_TRUNC|O_CREAT, 0666);
	void *mem;

	ftruncate(fd, clean + dirty);
	mem = mmap(0, clean + dirty, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	memset(mem, 0, clean + dirty);
	msync(mem, clean, MS_SYNC);
	ftruncate(fd, clean);
}


takes 45 seconds CPU time due to the O(clean * dirty) algorithm in
truncate_inode_pages().  The machine is locked up for the duration.
The patch reduces this to 20 milliseconds via an O(clean + dirty)
algorithm.


--- linux-2.4.5/mm/filemap.c	Mon May 28 13:31:49 2001
+++ linux-akpm/mm/filemap.c	Sun Jun 10 01:27:09 2001
@@ -273,15 +273,24 @@
 {
 	unsigned long start = (lstart + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
 	unsigned partial = lstart & (PAGE_CACHE_SIZE - 1);
+	int complete;
 
-repeat:
 	spin_lock(&pagecache_lock);
-	if (truncate_list_pages(&mapping->clean_pages, start, &partial))
-		goto repeat;
-	if (truncate_list_pages(&mapping->dirty_pages, start, &partial))
-		goto repeat;
-	if (truncate_list_pages(&mapping->locked_pages, start, &partial))
-		goto repeat;
+	do {
+		complete = 1;
+		while (truncate_list_pages(&mapping->clean_pages, start, &partial)) {
+			spin_lock(&pagecache_lock);
+			complete = 0;
+		}
+		while (truncate_list_pages(&mapping->dirty_pages, start, &partial)) {
+			spin_lock(&pagecache_lock);
+			complete = 0;
+		}
+		while (truncate_list_pages(&mapping->locked_pages, start, &partial)) {
+			spin_lock(&pagecache_lock);
+			complete = 0;
+		}
+	} while (!complete);
 	spin_unlock(&pagecache_lock);
 }
-
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