linux-kernel - Re: [PATCH v3 2/8] x86/mm: Change the leave

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Date:   Fri, 26 May 2017 09:25:28 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        Nadav Amit <nadav.amit@...il.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: [PATCH v3 2/8] x86/mm: Change the leave_mm() condition for
 local TLB flushes

On Thu, 2017-05-25 at 19:01 -0700, Andy Lutomirski wrote:
> On Thu, May 25, 2017 at 6:39 PM, Rik van Riel <riel@...hat.com>
> wrote:
> > On Thu, 2017-05-25 at 17:47 -0700, Andy Lutomirski wrote:
> > > 
> > > +++ b/arch/x86/mm/tlb.c
> > > @@ -311,7 +311,7 @@ void flush_tlb_mm_range(struct mm_struct *mm,
> > > unsigned long start,
> > >               goto out;
> > >       }
> > > 
> > > -     if (!current->mm) {
> > > +     if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
> > >               leave_mm(smp_processor_id());
> > 
> > Unless -mm changed leave_mm (I did not check), this
> > is not quite correct yet.
> > 
> > The reason is leave_mm (at least in the latest Linus
> > tree) ignores the cpu argument for one of its checks.
> > 
> > You should probably fix that in an earlier patch,
> > assuming you haven't already done so in -mm.
> > 
> > void leave_mm(int cpu)
> > {
> >         struct mm_struct *active_mm =
> > this_cpu_read(cpu_tlbstate.active_mm);
> >         if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
> >                 BUG();
> >         if (cpumask_test_cpu(cpu, mm_cpumask(active_mm))) {
> >                 cpumask_clear_cpu(cpu, mm_cpumask(active_mm));
> >                 load_cr3(swapper_pg_dir);
> 
> I agree it's odd, but what's the bug?  Both before and after,
> leave_mm
> needed to be called with cpu == smp_processor_id(), and
> smp_processor_id() warns if it's called in a preemptible context.

Indeed, you are right. Looking at too much code at once...