Date: Thu, 28 Oct 2004 11:21:09 -0700
From: Tim Hockin <thockin@hockin.org>
To: linux-kernel@vger.kernel.org
Subject: Re: Max groups one can be a member of linux/sched.h and NGROUPS_SMALL
Message-ID: <20041028182109.GA29258@hockin.org>
References: <20041028180230.GD10255@digitasaru.net>
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On Thu, Oct 28, 2004 at 01:02:30PM -0500, Joseph Pingenot wrote:
> #define NGROUPS_SMALL           32
> #define NGROUPS_PER_BLOCK       ((int)(PAGE_SIZE / sizeof(gid_t)))
> struct group_info {
>         int ngroups;
>         atomic_t usage;
>         gid_t small_block[NGROUPS_SMALL];
>         int nblocks;
>         gid_t *blocks[0];
> };

> So, it appears to hold 32 gids, but what is this blocks bit?  Is 32 the max
>   number of groups one can be a member of?

By default, every task has enough room for 32 gids.  If you need more than
32, it uses a dynamically allocated 2-d array, stored in blocks.  Always
use the GROUP_AT() macro, and you can treat it like a 1-d array.  I think
there is a hardlimit of 64k groups, but that is simply a #define that can
be changed.  You can have as many groups as you need, dynamically
allocated per-task, with CoW between tasks.  Sorted and bsearch()ed.
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