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 d74beb9f33a5f16d2965f11b275e401f225c949d ) 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>

2 files changed, 11 insertions, 11 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9c13be3a21e8..7c8ca05c3cae 100644
--- a/include/linux/sched.h
+++ b/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;
diff --git a/kernel/sys.c b/kernel/sys.c
index 2b8e2daa9d95..e3c08d4324de 100644
--- a/kernel/sys.c
+++ b/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;