On Mon, Apr 22, 2019 at 3:09 PM Alexey Dobriyan <adobri...@gmail.com> wrote: > > On Mon, Apr 22, 2019 at 07:30:40AM -0700, Andy Lutomirski wrote: > > > > > > > On Apr 22, 2019, at 3:34 AM, Ingo Molnar <mi...@kernel.org> wrote: > > > > > > > > > * Alexey Dobriyan <adobri...@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.
Saving 2kB of text by adding 8 bytes to thread_info seems rather dubious to me. You only need 256 tasks before you lose. My not-particularly-loaded laptop has 865 tasks right now. As a general principle, the mere existence of TIF_ADDR32 is a bug. The value of that flag is *wrong* under the 32-bit variant of CRIU. How about instead making some more progress toward getting rid of dubious TASK_SIZE users? I'm working on a little series to get rid of most of them. Meanwhile: it sure looks like a large fraction of the users are confused as to whether TASK_SIZE is the highest user address or the lowest non-user address.