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

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Date:	Thu, 16 Jul 2015 15:57:43 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Dave Hansen <dave@...1.net>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] x86, fpu: dynamically allocate 'struct fpu'

On Thu, Jul 16, 2015 at 3:42 PM, H. Peter Anvin <hpa@...or.com> wrote:
> 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.
>

In tip/x86/asm, entry_64.S only references thread_info in two places,
both in the syscall code.  I'm hoping that we can move that code into
C soon and that we can do the same thing to the 32-bit code.  If we do
that, then we could just move ti->flags into thread_struct.
Everything else should be easy, and then thread_info would be empty.

--Andy
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