linux-kernel - Re: [PATCH v4 2/2] ksm: provide support to use deferrable timers for scanner thread

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Date:	Wed, 10 Sep 2014 10:10:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Chintan Pandya <cpandya@...eaurora.org>, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 2/2] ksm: provide support to use deferrable timers for
 scanner thread

On Tue, Sep 09, 2014 at 01:14:50PM -0700, Hugh Dickins wrote:
> On Mon, 8 Sep 2014, Peter Zijlstra wrote:
> > >  		switch_mm(oldmm, mm, next);
> > > +		wake_ksm = ksm_switch(mm);
> > 
> > Is this the right mm?
> 
> It's next->mm, that's the one I intended (though the patch might
> be equally workable using prev->mm instead: given free rein, I'd
> have opted for hooking into both prev and next, but free rein is
> definitely not what should be granted around here!).
> 
> > We've just switched the stack,
> 
> I thought that came in switch_to() a few lines further down,
> but don't think it matters for this.

Ah, yes. Got my task and mm separation messed up.

> > so we're looing at next->mm when we switched away from current.
> > That might not exist anymore.
> 
> I fail to see how that can be.  Looking at the x86 switch_mm(),
> I can see it referencing (unsurprisingly!) both old and new mms
> at this point, and no reference to an mm is dropped before the
> ksm_switch().  oldmm (there called mm) is mmdropped later in
> finish_task_switch().

Well, see the above confusion about switch_mm vs switch_to :-/

So if this were switch_to(), we'd see next->mm as before the last
context switch. And since that switch fully happened, it would also
already have done the finish_task_switch() -> mmdrop().



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