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>

1 files changed, 10 insertions, 10 deletions

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;