Re: [PATCH v3 hmm 09/12] mm/hmm: Poison hmm_range during unregister

[Jason Gunthorpe <jgg@ziepe.ca>]

On Sat, Jun 15, 2019 at 07:17:26AM -0700, Christoph Hellwig wrote:
> > -	/* Sanity check this really should not happen. */
> > -	if (hmm == NULL || range->end <= range->start)
> > -		return;
> > -
> >  	mutex_lock(&hmm->lock);
> >  	list_del_rcu(&range->list);
> >  	mutex_unlock(&hmm->lock);
> >  
> >  	/* Drop reference taken by hmm_range_register() */
> > -	range->valid = false;
> >  	mmput(hmm->mm);
> >  	hmm_put(hmm);
> > -	range->hmm = NULL;
> > +
> > +	/*
> > +	 * The range is now invalid and the ref on the hmm is dropped, so
> > +         * poison the pointer.  Leave other fields in place, for the caller's
> > +         * use.
> > +         */
> > +	range->valid = false;
> > +	memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
> 
> Formatting seems to be messed up.  But again I don't see the value
> in the poisoning, just let normal linked list debugging do its work.
> The other cleanups looks fine to me.

tabs vs spaces, I fixed it. This one is more murky than the other - it
is to prevent the caller from using any of the range APIs after the
range is unregistered, but we could also safely use NULL here, I
think.

Jason
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