Messages in this thread | | | Date | Tue, 18 Dec 2007 16:24:53 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] kernel/sys.c : Get rid of expensive divides in groups_sort() |
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On Wed, 19 Dec 2007 01:14:33 +0100 Eric Dumazet <dada1@cosmosbay.com> wrote:
> 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. >
> diff --git a/include/linux/sched.h b/include/linux/sched.h > index ac3d496..725a491 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -775,7 +775,7 @@ 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)))
That was a dopey thing to do. Both PAGE_SIZE and sizeof() are unsigned and we went and cast it to a signed thing, even though it is a nonsense to ever consider a negative value of this.
> +#define NGROUPS_PER_BLOCK ((unsigned int)(PAGE_SIZE / sizeof(gid_t))) > struct group_info { > int ngroups; > atomic_t usage; > diff --git a/kernel/sys.c b/kernel/sys.c > index d1fe71e..091e58f 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -1148,7 +1148,7 @@ static int groups_to_user(gid_t __user *grouplist, > int count = group_info->ngroups; > > for (i = 0; i < group_info->nblocks; i++) { > - int cp_count = min(NGROUPS_PER_BLOCK, count); > + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count); > int off = i * NGROUPS_PER_BLOCK; > int len = cp_count * sizeof(*grouplist); > > @@ -1168,7 +1168,7 @@ static int groups_from_user(struct group_info *group_info, > int count = group_info->ngroups; > > for (i = 0; i < group_info->nblocks; i++) { > - int cp_count = min(NGROUPS_PER_BLOCK, count); > + int cp_count = min_t(int, NGROUPS_PER_BLOCK, count); > int off = i * NGROUPS_PER_BLOCK; > int len = cp_count * sizeof(*grouplist); >
We now have a jumble of signed and unsigned, stuck together with min_t sticky tape.
Please, take a closer look in there and see if there's anything which *needs* to be signed: is there anything which can actually, sensibly have a negative quantity? I bet there isn't, and I bet the code would be cleaner, faster and more obviously correct if it was converted to unsigned throughout.
I blame C. Negative quantities are rare, and C's default of treating scalars as signed was a mistake. Oh well.
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