kernel/sys.c: get rid of expensive divides in groups_sort()

groups_sort() can be quite long if user loads a large gid table.

This is because GROUP_AT(group_info, some_integer) uses an integer divide.
So having to do XXX thousand divides during one syscall can lead to very
high latencies.  (NGROUPS_MAX=65536)

In the past (25 Mar 2006), an analog problem was found in groups_search()
(commit d74beb9f ) and at that time I
changed some variables to unsigned int.

I believe that a more generic fix is to make sure NGROUPS_PER_BLOCK is
unsigned.
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -810,7 +810,7 @@ static inline int above_background_load(void)
 
 struct io_context;			/* See blkdev.h */
 #define NGROUPS_SMALL		32
-#define NGROUPS_PER_BLOCK	((int)(PAGE_SIZE / sizeof(gid_t)))
+#define NGROUPS_PER_BLOCK	((unsigned int)(PAGE_SIZE / sizeof(gid_t)))
 struct group_info {
 	int ngroups;
 	atomic_t usage;

kernel/sys.c

@@ -1145,16 +1145,16 @@ static int groups_to_user(gid_t __user *grouplist,
     struct group_info *group_info)
 {
 	int i;
-	int count = group_info->ngroups;
+	unsigned int count = group_info->ngroups;
 
 	for (i = 0; i < group_info->nblocks; i++) {
-		int cp_count = min(NGROUPS_PER_BLOCK, count);
-		int off = i * NGROUPS_PER_BLOCK;
-		int len = cp_count * sizeof(*grouplist);
+		unsigned int cp_count = min(NGROUPS_PER_BLOCK, count);
+		unsigned int len = cp_count * sizeof(*grouplist);
 
-		if (copy_to_user(grouplist+off, group_info->blocks[i], len))
+		if (copy_to_user(grouplist, group_info->blocks[i], len))
 			return -EFAULT;
 
+		grouplist += NGROUPS_PER_BLOCK;
 		count -= cp_count;
 	}
 	return 0;
@@ -1165,16 +1165,16 @@ static int groups_from_user(struct group_info *group_info,
     gid_t __user *grouplist)
 {
 	int i;
-	int count = group_info->ngroups;
+	unsigned int count = group_info->ngroups;
 
 	for (i = 0; i < group_info->nblocks; i++) {
-		int cp_count = min(NGROUPS_PER_BLOCK, count);
-		int off = i * NGROUPS_PER_BLOCK;
-		int len = cp_count * sizeof(*grouplist);
+		unsigned int cp_count = min(NGROUPS_PER_BLOCK, count);
+		unsigned int len = cp_count * sizeof(*grouplist);
 
-		if (copy_from_user(group_info->blocks[i], grouplist+off, len))
+		if (copy_from_user(group_info->blocks[i], grouplist, len))
 			return -EFAULT;
 
+		grouplist += NGROUPS_PER_BLOCK;
 		count -= cp_count;
 	}
 	return 0;