linux-kernel - Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm refcounting

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Date:   Sun, 29 Jul 2018 08:11:59 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>, will.daecon@....com,
        Catalin Marinas <catalin.marinas@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 10/10] mm,sched: conditionally skip lazy TLB mm
 refcounting

On Sat, 2018-07-28 at 21:21 -0700, Andy Lutomirski wrote:
> On Sat, Jul 28, 2018 at 2:53 PM, Rik van Riel <riel@...riel.com>
> wrote:
> > Conditionally skip lazy TLB mm refcounting. When an architecture
> > has
> > CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is used in
> > lazy TLB mode anywhere will get shot down from exit_mmap, and there
> > in no need to incur the cache line bouncing overhead of refcounting
> > a lazy TLB mm.
> 
> Unless I've misunderstood something, this patch results in idle tasks
> whose active_mm has been freed still having active_mm pointing at
> freed memory. 

Patch 9/10 is supposed to ensure that the lazy TLB CPUs get
switched to init_mm before an mm is freed. No CPU should ever
have its active_mm pointing at a freed mm.

Your message made me re-read the code, and now I realize that
leave_mm does not actually do that.

Looking at the other callers of leave_mm, I might not be the
only one surprised by that; xen_drop_mm_ref comes to mind.

I guess I should some code to leave_mm to have it actually
clear active_mm and call the conditional refcount drop helper
function.

Does that clear up the confusion?

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