Subject: Re: [PATCH 6/16] powerpc/mm: Split mmu_context handling v3

From: Arnd Bergmann <[email-masked]>
Date: Mon, 15 Dec 2008 16:43:44 +0100

On Monday 15 December 2008, Benjamin Herrenschmidt wrote:
> +/*
> + * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
> + * (virtual segment identifiers) for each context.  Although the
> + * hardware supports 24-bit VSIDs, and thus >1 million contexts,
> + * we only use 32,768 of them.  That is ample, since there can be
> + * at most around 30,000 tasks in the system anyway, and it means
> + * that we can use a bitmap to indicate which contexts are in use.
> + * Using a bitmap means that we entirely avoid all of the problems
> + * that we used to have when the context number overflowed,
> + * particularly on SMP systems.
> + *  -- paulus.
> + */

Didn't we lift the limit to 30,000 tasks at some point? The comment
in linux/threads.h mentions that the PID space goes up to 4 million.
What does actually happen when we increase pid_max to beyond 32768
on those systems and try to use them? Is there another limit in place?

	Arnd <><