linux-kernel - Re: [RFC 09/10] x86/mm: Rework lazy TLB to track the actual loaded mm

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Date:   Wed, 10 May 2017 10:24:25 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Nadav Amit <namit@...are.com>, Michal Hocko <mhocko@...e.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [RFC 09/10] x86/mm: Rework lazy TLB to track the actual loaded mm


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Wed, 10 May 2017, Ingo Molnar wrote:
> > 
> > * Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> > > On Sun, 7 May 2017, Andy Lutomirski wrote:
> > > >  /* context.lock is held for us, so we don't need any locking. */
> > > >  static void flush_ldt(void *current_mm)
> > > >  {
> > > > +	struct mm_struct *mm = current_mm;
> > > >  	mm_context_t *pc;
> > > >  
> > > > -	if (current->active_mm != current_mm)
> > > > +	if (this_cpu_read(cpu_tlbstate.loaded_mm) != current_mm)
> > > 
> > > While functional correct, this really should compare against 'mm'.
> > > 
> > > >  		return;
> > > >  
> > > > -	pc = &current->active_mm->context;
> > > > +	pc = &mm->context;
> > 
> > So this appears to be the function:
> > 
> >  static void flush_ldt(void *current_mm)
> >  {
> >         struct mm_struct *mm = current_mm;
> >         mm_context_t *pc;
> > 
> >         if (this_cpu_read(cpu_tlbstate.loaded_mm) != current_mm)
> >                 return;
> > 
> >         pc = &mm->context;
> >         set_ldt(pc->ldt->entries, pc->ldt->size);
> >  }
> > 
> > why not rename 'current_mm' to 'mm' and remove the 'mm' local variable?
> 
> Because you cannot dereference a void pointer, i.e. &mm->context ....

Indeed, doh! The naming totally confused me. The way I'd write it is the canonical 
form for such callbacks:

	static void flush_ldt(void *data)
	{
		struct mm_struct *mm = data;

... which beyond unconfusing me would probably also have prevented any accidental 
use of the 'current_mm' callback argument.

Thanks,

	Ingo