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Date:   Tue, 23 Apr 2019 01:09:48 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Ingo Molnar <mingo@kernel.org>, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, linux-kernel@vger.kernel.org,
        x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH] x86_64: uninline TASK_SIZE
Message-ID: <20190422220948.GB26031@avx2>
References: <20190421160600.GA31092@avx2>
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On Mon, Apr 22, 2019 at 07:30:40AM -0700, Andy Lutomirski wrote:
> 
> 
> > On Apr 22, 2019, at 3:34 AM, Ingo Molnar <mingo@kernel.org> wrote:
> > 
> > 
> > * Alexey Dobriyan <adobriyan@gmail.com> wrote:
> > 
> >>>>> +++ b/arch/x86/kernel/task_size_64.c
> >>>>> @@ -0,0 +1,9 @@
> >>>>> +#include <linux/export.h>
> >>>>> +#include <linux/sched.h>
> >>>>> +#include <linux/thread_info.h>
> >>>>> +
> >>>>> +unsigned long _task_size(void)
> >>>>> +{
> >>>>> +    return test_thread_flag(TIF_ADDR32) ? IA32_PAGE_OFFSET :
> >>>> TASK_SIZE_MAX;
> >>>>> +}
> >>>>> +EXPORT_SYMBOL(_task_size);
> >>>> 
> >>>> Good idea - but instead of adding yet another compilation unit, why not
> >>>> 
> >>>> stick _task_size() into arch/x86/kernel/process_64.c, which is the 
> >>>> canonical place for process management related arch functions?
> >>>> 
> >>>> Thanks,
> >>>> 
> >>>>    Ingo
> >>> 
> >>> Better yet... since TIF_ADDR32 isn't something that changes randomly, 
> >>> perhaps this should be a separate variable?
> >> 
> >> Maybe. I only thought about putting every 32-bit related flag under 
> >> CONFIG_COMPAT to further eradicate bloat (and force everyone else to 
> >> keep an eye on it, ha-ha).
> > 
> > Basically TIF_ADDR32 is only set for a task if set_personality_ia32() is 
> > called, which function is called in the following circumstances:
> > 
> > - arch/x86/ia32/ia32_aout.c:load_aout_binary()
> > 
> >   This is in exec(), when a new binary is loaded for the current task, 
> >   via search_binary_handler() and exec_binprm(). Ordering is 
> >   synchronous, AFAICS there can be no race between TASK_SIZE users and 
> >   the set_personality_ia32() call which is always for the current task.
> > 
> > - in COMPAT_SET_PERSONALITY(), which through macro detours ends up being 
> >   in SET_PERSONALITY2(), which is used in fs/compat_binfmt_elf.c's 
> >   load_elf_binary(), used in a similar fashion in exec() as the AOUT 
> >   case above. One particular macro detour of note is that 
> >   fs/compat_binfmt_elf.c #includes fs/binfmt_elf.c and re-defines the 
> >   personality setting method to map to set_personality_ia32().
> > 
> > When set_personality_ia32() is called then TIF_ADDR32 is set 
> > unconditionally, without any Kconfig variations.
> > 
> > TIF_ADDR32 is cleared:
> > 
> > - In set_personality_64bit(), when a 64-bit binary is loaded via 
> >   fs/binfmt_elf.c.
> > 
> > - It also defaults to clear in the init task, which is inherited by the 
> >   initial kernel threads and any user-space task they might end up 
> >   executing.
> > 
> > So the conclusion is that IMO we can safely put TASK_SIZE into a new 
> > thread_info()->task_size field, and:
> > 
> > - change ->task_size to the 32-bit address space in 
> >   set_personality_ia32()
> > 
> > - change ->task_size to teh 64-bit address space in the init task and in 
> >   set_personality_64bit().
> > 
> > This should cover it I think, unless I missed something.
> > 
> 
> Are there really enough TASK_SIZE users to justify any of this?

Saving 2KB on a defconfig is quite a lot.
If put into thread_info, ->task_size can be pulled using just RAX which
in turn allows to do

	asm volatile "call %P" ...  "=a" (...)

saving even more space.

But it is late here so don't quote me.