Re: [PATCH v3 2/8] x86/mm: Change the leave_mm() condition for local TLB flushes

From: Rik van Riel <riel@redhat.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bpetkov@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mel Gorman <mgorman@suse.de>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Nadav Amit <nadav.amit@gmail.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Nadav Amit <namit@vmware.com>, Michal Hocko <mhocko@suse.com>,
	Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [PATCH v3 2/8] x86/mm: Change the leave_mm() condition for local TLB flushes
Date: Fri, 26 May 2017 09:25:28 -0400	[thread overview]
Message-ID: <1495805128.29205.64.camel@redhat.com> (raw)
In-Reply-To: <CALCETrVDpWL4kQbxNVWBX-OKuThoaYaqefbKY-dD0A2y4BgNfA@mail.gmail.com>

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@redhat.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,
> > > A A A A A A A A A A A A A A goto out;
> > > A A A A A A }
> > > 
> > > -A A A A A if (!current->mm) {
> > > +A A A A A if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
> > > A A A A A A A A A A A A A A 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)
> > {
> > A A A A A A A A struct mm_struct *active_mm =
> > this_cpu_read(cpu_tlbstate.active_mm);
> > A A A A A A A A if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
> > A A A A A A A A A A A A A A A A BUG();
> > A A A A A A A A if (cpumask_test_cpu(cpu, mm_cpumask(active_mm))) {
> > A A A A A A A A A A A A A A A A cpumask_clear_cpu(cpu, mm_cpumask(active_mm));
> > A A A A A A A A A A A A A A A A load_cr3(swapper_pg_dir);
> 
> I agree it's odd, but what's the bug?A A 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...

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