Re: [PATCH] x86_64: uninline TASK_SIZE

Alexey Dobriyan
2019-04-22T22:09:54Z

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.