kernel/sys.c: get rid of expensive divides in groups_sort()
Eric Dumazet [Wed, 6 Feb 2008 09:37:56 +0000 (01:37 -0800)]
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>

include/linux/sched.h
kernel/sys.c

index 9c13be3..7c8ca05 100644 (file)
@@ -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;
index 2b8e2da..e3c08d4 100644 (file)
@@ -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;