linux-kernel - Re: [PATCH 2/7] mmu_notifier: keep track of active invalidation ranges v2

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Date:	Thu, 25 Dec 2014 10:29:44 +0200
From:	Haggai Eran <haggaie@...lanox.com>
To:	<j.glisse@...il.com>, <akpm@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	<joro@...tes.org>, Mel Gorman <mgorman@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Johannes Weiner" <jweiner@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	"Rik van Riel" <riel@...hat.com>, Dave Airlie <airlied@...hat.com>,
	Brendan Conoboy <blc@...hat.com>,
	Joe Donohue <jdonohue@...hat.com>,
	Duncan Poole <dpoole@...dia.com>,
	Sherry Cheung <SCheung@...dia.com>,
	Subhash Gutti <sgutti@...dia.com>,
	John Hubbard <jhubbard@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Lucien Dunning <ldunning@...dia.com>,
	"Cameron Buschardt" <cabuschardt@...dia.com>,
	Arvind Gopalakrishnan <arvindg@...dia.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Liran Liss <liranl@...lanox.com>,
	Roland Dreier <roland@...estorage.com>,
	Ben Sander <ben.sander@....com>,
	Greg Stoner <Greg.Stoner@....com>,
	John Bridgman <John.Bridgman@....com>,
	Michael Mantor <Michael.Mantor@....com>,
	"Paul Blinzer" <Paul.Blinzer@....com>,
	Laurent Morichetti <Laurent.Morichetti@....com>,
	Alexander Deucher <Alexander.Deucher@....com>,
	Oded Gabbay <Oded.Gabbay@....com>,
	Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 2/7] mmu_notifier: keep track of active invalidation ranges
 v2

On 22/12/2014 18:48, j.glisse@...il.com wrote:
>  static inline void mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> -						       unsigned long start,
> -						       unsigned long end,
> -						       enum mmu_event event)
> +						       struct mmu_notifier_range *range)
>  {
> +	/*
> +	 * Initialize list no matter what in case a mmu_notifier register after
> +	 * a range_start but before matching range_end.
> +	 */
> +	INIT_LIST_HEAD(&range->list);

I don't see how can an mmu_notifier register after a range_start but
before a matching range_end. The mmu_notifier registration locks all mm
locks, and that should prevent any invalidation from running, right?

>  	if (mm_has_notifiers(mm))
> -		__mmu_notifier_invalidate_range_start(mm, start, end, event);
> +		__mmu_notifier_invalidate_range_start(mm, range);
>  }

...

>  void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> -					   unsigned long start,
> -					   unsigned long end,
> -					   enum mmu_event event)
> +					   struct mmu_notifier_range *range)
>  
>  {
>  	struct mmu_notifier *mn;
> @@ -185,21 +183,36 @@ void __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
>  	id = srcu_read_lock(&srcu);
>  	hlist_for_each_entry_rcu(mn, &mm->mmu_notifier_mm->list, hlist) {
>  		if (mn->ops->invalidate_range_start)
> -			mn->ops->invalidate_range_start(mn, mm, start,
> -							end, event);
> +			mn->ops->invalidate_range_start(mn, mm, range);
>  	}
>  	srcu_read_unlock(&srcu, id);
> +
> +	/*
> +	 * This must happen after the callback so that subsystem can block on
> +	 * new invalidation range to synchronize itself.
> +	 */
> +	spin_lock(&mm->mmu_notifier_mm->lock);
> +	list_add_tail(&range->list, &mm->mmu_notifier_mm->ranges);
> +	mm->mmu_notifier_mm->nranges++;
> +	spin_unlock(&mm->mmu_notifier_mm->lock);
>  }
>  EXPORT_SYMBOL_GPL(__mmu_notifier_invalidate_range_start);

Don't you have a race here because you add the range struct after the
callback?

-------------------------------------------------------------------------
Thread A                    | Thread B
-------------------------------------------------------------------------
call mmu notifier callback  |
  clear SPTE                |
                            | device page fault
                            |   mmu_notifier_range_is_valid returns true
                            |   install new SPTE
add event struct to list    |
mm clears/modifies the PTE  |
-------------------------------------------------------------------------

So we are left with different entries in the host page table and the
secondary page table.

I would think you'd want the event struct to be added to the list before
the callback is run.

Best regards,
Haggai


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