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Commit 1bf47346 authored by Eric Dumazet's avatar Eric Dumazet Committed by Linus Torvalds
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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 d74beb9f ) 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: default avatarEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 6b2fb3c6

include/linux/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -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;

kernel/sys.c

+10 −10
Original line number Diff line number Diff line
@@ -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;