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Subject: Re: [PATCH] x86_64: uninline TASK_SIZE
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190422103449.GA75723@gmail.com>
Date:   Mon, 22 Apr 2019 07:30:40 -0700
Cc:     Alexey Dobriyan <adobriyan@gmail.com>, 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>
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Message-Id: <BFD71398-0BDC-4045-91E9-C70A22FF6E73@amacapital.net>
References: <20190421160600.GA31092@avx2> <20190421182842.GD35603@gmail.com> <8B42CD57-9343-4234-A96D-80337BFFDF0E@zytor.com> <20190421211007.GA30444@avx2> <20190422103449.GA75723@gmail.com>
To:     Ingo Molnar <mingo@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> 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?