Date:	Wed, 4 Mar 2009 17:14:29 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Nicolas Pitre <nico@....org>
Cc:	lkml <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [RFC] atomic highmem kmap page pinning

On Wed, 04 Mar 2009 00:58:13 -0500 (EST)
Nicolas Pitre <nico@....org> wrote:

> I've implemented highmem for ARM.  Yes, some ARM machines do have lots 
> of memory...
> 
> The problem is that most ARM machines have a non IO coherent cache, 
> meaning that the dma_map_* set of functions must clean and/or invalidate 
> the affected memory manually.  And because the majority of those 
> machines have a VIVT cache, the cache maintenance operations must be 
> performed using virtual addresses.
> 
> In dma_map_page(), an highmem pages could still be mapped and cached 
> even after kunmap() was called on it.  As long as highmem pages are 
> mapped, page_address(page) is non null and we can use that to 
> synchronize the cache.
> It is unlikely but still possible for kmap() to race and recycle the 
> obtained virtual address above, and use it for another page though.  In 
> that case, the new mapping could end up with dirty cache lines for 
> another page, and the unsuspecting cache invalidation loop in 
> dma_map_page() won't notice resulting in data loss.  Hence the need for 
> some kind of kmap page pinning which can be used in any context, 
> including IRQ context.
> 
> This is a RFC patch implementing the necessary part in the core code, as 
> suggested by RMK. Please comment.

I am not sure if i understand your concern totally.
I can understand it can be recycled. but Why is it racing ?
Now, kmap semantic is that it can't be called in interrupt context. 

As far as I understand, To make irq_disable to prevent this problem is rather big cost.

I think it would be better to make page_address can return null in that case
where pkmap_count is less than one or it's not previous page mapping.


> diff --git a/mm/highmem.c b/mm/highmem.c
> index b36b83b..548ca77 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -113,9 +113,9 @@ static void flush_all_zero_pkmaps(void)
>   */
>  void kmap_flush_unused(void)
>  {
> -	spin_lock(&kmap_lock);
> +	spin_lock_irq(&kmap_lock);
>  	flush_all_zero_pkmaps();
> -	spin_unlock(&kmap_lock);
> +	spin_unlock_irq(&kmap_lock);
>  }
>  
>  static inline unsigned long map_new_virtual(struct page *page)
> @@ -145,10 +145,10 @@ start:
>  
>  			__set_current_state(TASK_UNINTERRUPTIBLE);
>  			add_wait_queue(&pkmap_map_wait, &wait);
> -			spin_unlock(&kmap_lock);
> +			spin_unlock_irq(&kmap_lock);
>  			schedule();
>  			remove_wait_queue(&pkmap_map_wait, &wait);
> -			spin_lock(&kmap_lock);
> +			spin_lock_irq(&kmap_lock);
>  
>  			/* Somebody else might have mapped it while we slept */
>  			if (page_address(page))
> @@ -184,19 +184,43 @@ void *kmap_high(struct page *page)
>  	 * For highmem pages, we can't trust "virtual" until
>  	 * after we have the lock.
>  	 */
> -	spin_lock(&kmap_lock);
> +	spin_lock_irq(&kmap_lock);
>  	vaddr = (unsigned long)page_address(page);
>  	if (!vaddr)
>  		vaddr = map_new_virtual(page);
>  	pkmap_count[PKMAP_NR(vaddr)]++;
>  	BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 2);
> -	spin_unlock(&kmap_lock);
> +	spin_unlock_irq(&kmap_lock);
>  	return (void*) vaddr;
>  }
>  
>  EXPORT_SYMBOL(kmap_high);
>  
>  /**
> + * kmap_high_get - pin a highmem page into memory
> + * @page: &struct page to pin
> + *
> + * Returns the page's current virtual memory address, or NULL if no mapping
> + * exists.  When and only when a non null address is returned then a
> + * matching call to kunmap_high() is necessary.
> + *
> + * This can be called from interrupt context.
> + */
> +void *kmap_high_get(struct page *page)
> +{
> +	unsigned long vaddr, flags;
> +
> +	spin_lock_irqsave(&kmap_lock, flags);
> +	vaddr = (unsigned long)page_address(page);
> +	if (vaddr) {
> +		BUG_ON(pkmap_count[PKMAP_NR(vaddr)] < 1);
> +		pkmap_count[PKMAP_NR(vaddr)]++;
> +	}
> +	spin_unlock_irqrestore(&kmap_lock, flags);
> +	return (void*) vaddr;
> +}
> +
> +/**
>   * kunmap_high - map a highmem page into memory
>   * @page: &struct page to unmap
>   */
> @@ -204,9 +228,10 @@ void kunmap_high(struct page *page)
>  {
>  	unsigned long vaddr;
>  	unsigned long nr;
> +	unsigned long flags;
>  	int need_wakeup;
>  
> -	spin_lock(&kmap_lock);
> +	spin_lock_irqsave(&kmap_lock, flags);
>  	vaddr = (unsigned long)page_address(page);
>  	BUG_ON(!vaddr);
>  	nr = PKMAP_NR(vaddr);
> @@ -232,7 +257,7 @@ void kunmap_high(struct page *page)
>  		 */
>  		need_wakeup = waitqueue_active(&pkmap_map_wait);
>  	}
> -	spin_unlock(&kmap_lock);
> +	spin_unlock_irqrestore(&kmap_lock, flags);
>  
>  	/* do wake-up, if needed, race-free outside of the spin lock */
>  	if (need_wakeup)
> 
> 
> Nicolas
> --
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-- 
Kinds Regards
Minchan Kim
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