Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 16, 2015 at 12:14:37PM -0700, Dave Hansen wrote:
> +++ b/arch/x86/kernel/fpu/init.c	2015-07-16 12:02:15.284280976 -0700
> @@ -136,6 +136,45 @@ static void __init fpu__init_system_gene
>  unsigned int xstate_size;
>  EXPORT_SYMBOL_GPL(xstate_size);
>  
> +#define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER)	\
> +	BUILD_BUG_ON((sizeof(TYPE) -			\
> +			offsetof(TYPE, MEMBER) -	\
> +			sizeof(((TYPE *)0)->MEMBER)) > 	\
> +			0)				\
> +
> +/*
> + * We append the 'struct fpu' to the task_struct.
> + */
> +int __weak arch_task_struct_size(void)
> +{
> +	int task_size = sizeof(struct task_struct);
> +
> +	/*
> +	 * Subtract off the static size of the register state.
> +	 * It potentially has a bunch of padding.
> +	 */
> +	task_size -= sizeof(((struct task_struct *)0)->thread.fpu.state);
> +
> +	/*
> +	 * Add back the dynamically-calculated register state
> +	 * size.
> +	 */
> +	task_size += xstate_size;
> +
> +	/*
> +	 * We dynamically size 'struct fpu', so we require that
> +	 * it be at the end of 'thread_struct' and that
> +	 * 'thread_struct' be at the end of 'task_struct'.  If
> +	 * you hit a compile error here, check the structure to
> +	 * see if something got added to the end.
> +	 */
> +	CHECK_MEMBER_AT_END_OF(struct fpu, state);
> +	CHECK_MEMBER_AT_END_OF(struct thread_struct, fpu);
> +	CHECK_MEMBER_AT_END_OF(struct task_struct, thread);
> +
> +	return task_size;
> +}

Since you want these invariants true at all times, maybe put the
BUILD_BUG_ON() in generic code instead of x86 specific? That way people
poking at other archs are less likely to accidentally break your stuff.