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

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Date:	Thu, 16 Jul 2015 15:42:24 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave@...1.net>
CC:	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	peterz@...radead.org, bp@...en8.de, luto@...capital.net,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'

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


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