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

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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 d74beb9f33 ) 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>
This commit is contained in:
Eric Dumazet 2008-02-06 01:37:56 -08:00 committed by Linus Torvalds
parent 6b2fb3c658
commit 1bf47346d7
2 changed files with 11 additions and 11 deletions
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2
include/linux/sched.h
View 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;

20
kernel/sys.c
View 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;