linux-kernel - Re: Re: [PATCH 05/15] HMM: introduce heterogeneous memory management v4.

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Date:	Tue, 4 Aug 2015 10:17:02 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	GIRISH K S <ks.giri@...sung.com>
Cc:	Girish KS <girishks2000@...il.com>,
	J??e Glisse <jglisse@...hat.com>,
	Christophe Harle <charle@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Dave Airlie <airlied@...hat.com>,
	Arvind Gopalakrishnan <arvindg@...dia.com>,
	Jatin Kumar <jakumar@...dia.com>,
	"joro@...tes.org" <joro@...tes.org>,
	Greg Stoner <Greg.Stoner@....com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Rik van Riel <riel@...hat.com>,
	Paul Blinzer <Paul.Blinzer@....com>,
	Lucien Dunning <ldunning@...dia.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Haggai Eran <haggaie@...lanox.com>,
	Michael Mantor <Michael.Mantor@....com>,
	Laurent Morichetti <Laurent.Morichetti@....com>,
	Larry Woodman <lwoodman@...hat.com>,
	John Hubbard <jhubbard@...dia.com>,
	Brendan Conoboy <blc@...hat.com>,
	John Bridgman <John.Bridgman@....com>,
	Subhash Gutti <sgutti@...dia.com>,
	Roland Dreier <roland@...estorage.com>,
	Duncan Poole <dpoole@...dia.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Alexander Deucher <Alexander.Deucher@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Leonid Shamis <Leonid.Shamis@....com>,
	Sherry Cheung <SCheung@...dia.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Shachar Raindel <raindel@...lanox.com>,
	Liran Liss <liranl@...lanox.com>,
	Ben Sander <ben.sander@....com>,
	Joe Donohue <jdonohue@...hat.com>,
	Mel Gorman <mgorman@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Re: [PATCH 05/15] HMM: introduce heterogeneous memory management
 v4.

On Mon, Aug 03, 2015 at 12:21:14PM +0000, GIRISH K S wrote:
> On Mon, Aug 03, 2015 at 01:20:13PM +0530, Girish KS wrote:
> > On 18-Jul-2015 12:47 am, "JÃƒÂ¯Ã‚Â¿Ã‚Â½ÃƒÂ¯Ã‚Â¿Ã‚Â½e Glisse" wrote:
> > >
> 
> [...]
> 
> > > +int hmm_mirror_register(struct hmm_mirror *mirror)
> > > +{
> > > +       struct mm_struct *mm = current->mm;
> > > +       struct hmm *hmm = NULL;
> > > +       int ret = 0;
> > > +
> > > +       /* Sanity checks. */
> > > +       BUG_ON(!mirror);
> > > +       BUG_ON(!mirror->device);
> > > +       BUG_ON(!mm);
> > > +
> > > +       /*
> > > +        * Initialize the mirror struct fields, the mlist init and del
> > dance is
> > > +        * necessary to make the error path easier for driver and for hmm.
> > > +        */
> > > +       kref_init(&mirror->kref);
> > > +       INIT_HLIST_NODE(&mirror->mlist);
> > > +       INIT_LIST_HEAD(&mirror->dlist);
> > > +       spin_lock(&mirror->device->lock);
> > > +       list_add(&mirror->dlist, &mirror->device->mirrors);
> > > +       spin_unlock(&mirror->device->lock);
> > > +
> > > +       down_write(&mm->mmap_sem);
> > > +
> > > +       hmm = mm->hmm ? hmm_ref(hmm) : NULL;
> > 
> > Instead of hmm mm->hmm would be the right param to be passed.  Here even
> > though mm->hmm is true hmm_ref returns NULL. Because hmm is not updated
> > after initialization in the beginning.
> 
> ENOPARSE ? While this can be simplified to hmm = hmm_ref(mm->hmm); I do not
> see what you mean. The mm struct might already have a valid hmm field set,
> and that valid hmm struct might also already be in the process of being
> destroy. So hmm_ref() might either return the same hmm pointer if the hmm
> object is not about to be release or NULL. But at this point there is no
> certainty on the return value of hmm_ref().
> 
> I didn't mean hmm = hmm_ref(mm->hmm);. I ll try to put it in a better way.
> The hmm local variable is initialized to NULL in the start of the function
> (struct hmm *hmm = NULL;), and this is not modified till it is passed to
> hmm_ref.  So hmm_ref would always return a NULL irrespective of mm->hmm is
> NULL or valid address.  
> So  the statement hmm = mm->hmm ? hmm_ref(hmm) : NULL; should be replaced
> as hmm = mm->hmm ? hmm_ref(mm->hmm) : NULL;. 

Oh yeah typo probably outcome of many patch reorg i did.

Cheers,
Jérôme
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