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

H. Peter Anvin
2015-07-16T22:43:25Z

On 07/16/2015 12:14 PM, Dave Hansen wrote:
> The FPU rewrite removed the dynamic allocations of 'struct fpu'.
> But, this potentially wastes massive amounts of memory (2k per
> task on systems that do not have AVX-512 for instance).
>
> Instead of having a separate slab, this patch just appends the
> space that we need to the 'task_struct' which we dynamically
> allocate already. This saves from doing an extra slab allocation
> at fork(). The only real downside here is that we have to stick
> everything and the end of the task_struct. But, I think the
> BUILD_BUG_ON()s I stuck in there should keep that from being too
> fragile.
>
> This survives a quick build and boot in a VM. Does anyone see any
> real downsides to this?
>

No. I have also long advocated for merging task_struct and thread_info
into a common structure and get it off the stack; it would improve
security and avoid weird corner cases in the irqstack handling.

-hpa