[PATCH RFC] arm64: introduce mm_context_t flags

[ynorov@caviumnetworks.com (Yury Norov)]

On Wed, Aug 02, 2017 at 05:39:01PM +0100, Catalin Marinas wrote:
> Hi Yury,
> 
> On Mon, Jul 31, 2017 at 05:48:25PM +0300, Yury Norov wrote:
> > In patch 06beb72fbe23e ("arm64: introduce mm context flag to keep 32 bit task
> > information") you introduce the field flags but use it only for a single flag -
> > TIF_32BIT. It looks hacky to me for three reasons:
> >  - The flag is introduced for the case where it's impossible to get the thread
> >    info structure for the thread associated with mm. So thread_info flags (TIF)
> >    may also be unavailable at place. This is not the case for the only existing
> >    user of if - uprobes, but in general this approach requires to include thread
> >    headers in mm code, which may become unwanted dependency.
> >  - New flag, if it uses TIF bits, for consistency should for example set/clear
> >    TIF_32BIT_AARCH64 for ILP32 tasks. And to be completely consistent, with
> >    current approach we'd mirror thread_info flags to mm_context flags. And keep
> >    it syncronized.
> >  - If we start using TIF flags here, we cannot easily add new mm_context
> >    specific bits because they may mess with TIF ones.
> > 
> > I think that this is not what was intended when you added new field in
> > mm_context_t.
> 
> TIF_32BIT was handy at the time but it indeed denotes AArch32 user
> task. For ILP32 we wouldn't need to set this bit since the instruction
> set is A64 and uprobe should support it (though not sure anyone tried).
> I noticed in your patch introducing binfmt_ilp32.c that SET_PERSONALITY
> actually sets this flag in the mm context.

Depending on what will be decided here, I'll change ilp32 patch
accordingly.

> As with the TIF bits, the PERSONALITY macros become more complicated
> with more bits to set/clear. Since we don't have any plans for other mm
> context flags (independent of TIF), should we simply rename it to
> thread_flags and just copy the thread_info flags:
> 
> 	current->mm->context.thread_flags = current_thread_info()->flags;

This will also work. But it may raise questions to one who reads the
code.
- if mm_context needs the threads flags, why you copy it? Why not to
  move flags to the mm_context_t? It is always available for
  thread_info users.
- for multithreaded applications there might be different set of bits
  in the flags field in different theads. So what exactly will be in
  context.thread_flags is a matter of case, and you'd explain somehow
  which bits are reliable, and which are not.
- It doesn't sound convincing to me that there are no other candidates
  for mm_context.flags bits. 6 month ago we didn't need the flags at all.
  ARM64 is under intensive development, and it's highly probable that
  candidates may appear one day.
  
I don't like to add new types as well, but at the case I think, this is
the most straightforward and simple way to introduce new set of bits for
new bitfield. And also less questionable in maintenance perspective.

Yury