From: Eric Dumazet 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 Signed-off-by: Andrew Morton --- include/linux/sched.h | 2 +- kernel/sys.c | 20 ++++++++++---------- 2 files changed, 11 insertions(+), 11 deletions(-) diff -puN include/linux/sched.h~kernel-sysc-get-rid-of-expensive-divides-in-groups_sort include/linux/sched.h --- a/include/linux/sched.h~kernel-sysc-get-rid-of-expensive-divides-in-groups_sort +++ a/include/linux/sched.h @@ -810,7 +810,7 @@ static inline int above_background_load( 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 -puN kernel/sys.c~kernel-sysc-get-rid-of-expensive-divides-in-groups_sort kernel/sys.c --- a/kernel/sys.c~kernel-sysc-get-rid-of-expensive-divides-in-groups_sort +++ a/kernel/sys.c @@ -1145,16 +1145,16 @@ static int groups_to_user(gid_t __user * 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 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; _