linux-kernel - Re: [PATCH 28/83] mm: Change timing of notification to IOMMUs about a page to be invalidated

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Date:	Thu, 10 Jul 2014 15:34:29 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Joerg Roedel <joro@...tes.org>
cc:	Oded Gabbay <oded.gabbay@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Airlie <airlied@...ux.ie>,
	Alex Deucher <alexander.deucher@....com>,
	Jerome Glisse <j.glisse@...il.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	John Bridgman <John.Bridgman@....com>,
	Andrew Lewycky <Andrew.Lewycky@....com>,
	linux-mm <linux-mm@...ck.org>, Oded Gabbay <oded.gabbay@....com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jerome Glisse <jglisse@...hat.com>,
	Jianyu Zhan <nasa4836@...il.com>
Subject: Re: [PATCH 28/83] mm: Change timing of notification to IOMMUs about
 a page to be invalidated

On Fri, 11 Jul 2014, Joerg Roedel wrote:
> On Fri, Jul 11, 2014 at 12:53:26AM +0300, Oded Gabbay wrote:
> >  mm/rmap.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 196cd0c..73d4c3d 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1231,13 +1231,17 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >  	} else
> >  		dec_mm_counter(mm, MM_FILEPAGES);
> >  
> > +	pte_unmap_unlock(pte, ptl);
> > +
> > +	mmu_notifier_invalidate_page(vma, address, event);
> > +
> >  	page_remove_rmap(page);
> >  	page_cache_release(page);
> >  
> > +	return ret;
> > +
> >  out_unmap:
> >  	pte_unmap_unlock(pte, ptl);
> > -	if (ret != SWAP_FAIL && !(flags & TTU_MUNLOCK))
> > -		mmu_notifier_invalidate_page(vma, address, event);
> >  out:
> >  	return ret;
> 
> I think there is no bug. In that function the page is just unmapped,
> removed from the rmap (page_remove_rmap), and the LRU list
> (page_cache_release). The page itself is not released in this function,
> so the call mmu_notifier_invalidate_page() at the end is fine.

Agreed, nothing to fix here: the try_to_unmap() callers must hold
their own reference to the page.  If they did not, how could they
be sure that this is a page which is appropriate to unmap?

(Nit: we don't actually take a separate reference for the LRU list:
the page_cache_release above corresponds to the reference in the
pte which has just been removed.)

Hugh
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