linux-kernel - Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct

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Date:   Wed, 20 Feb 2019 19:15:57 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Ralph Campbell <rcampbell@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct

On Wed, Feb 20, 2019 at 04:06:50PM -0800, John Hubbard wrote:
> On 2/20/19 3:59 PM, Jerome Glisse wrote:
> > On Wed, Feb 20, 2019 at 03:47:50PM -0800, John Hubbard wrote:
> > > On 1/29/19 8:54 AM, jglisse@...hat.com wrote:
> > > > From: Jérôme Glisse <jglisse@...hat.com>
> > > > 
> > > > Every time i read the code to check that the HMM structure does not
> > > > vanish before it should thanks to the many lock protecting its removal
> > > > i get a headache. Switch to reference counting instead it is much
> > > > easier to follow and harder to break. This also remove some code that
> > > > is no longer needed with refcounting.
> > > 
> > > Hi Jerome,
> > > 
> > > That is an excellent idea. Some review comments below:
> > > 
> > > [snip]
> > > 
> > > >    static int hmm_invalidate_range_start(struct mmu_notifier *mn,
> > > >    			const struct mmu_notifier_range *range)
> > > >    {
> > > >    	struct hmm_update update;
> > > > -	struct hmm *hmm = range->mm->hmm;
> > > > +	struct hmm *hmm = hmm_get(range->mm);
> > > > +	int ret;
> > > >    	VM_BUG_ON(!hmm);
> > > > +	/* Check if hmm_mm_destroy() was call. */
> > > > +	if (hmm->mm == NULL)
> > > > +		return 0;
> > > 
> > > Let's delete that NULL check. It can't provide true protection. If there
> > > is a way for that to race, we need to take another look at refcounting.
> > 
> > I will do a patch to delete the NULL check so that it is easier for
> > Andrew. No need to respin.
> 
> (Did you miss my request to make hmm_get/hmm_put symmetric, though?)

Went over my mail i do not see anything about symmetric, what do you
mean ?

Cheers,
Jérôme