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 15:42:27 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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

On Wed, May 10, 2017 at 1:24 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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.
>
>

void *data and void *info both seem fairly common in the kernel.  How
about my personal favorite for non-kernel work, though: void *mm_void?
 It documents what the parameter means and avoids the confusion.

--Andy