linux-kernel - Re: [RFC] atomic highmem kmap page pinning

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Message-id: <alpine.LFD.2.00.0903042129140.5511@xanadu.home>
Date:	Wed, 04 Mar 2009 21:37:43 -0500 (EST)
From:	Nicolas Pitre <nico@....org>
To:	Minchan Kim <minchan.kim@...il.com>
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 Thu, 5 Mar 2009, Minchan Kim wrote:

> I thought kmap and dma_map_page usage was following.
> 
> kmap(page);
> ...
> dma_map_page(...)
>   invalidate_cache_line
> 
> kunmap(page);
> 
> In this case, how do pkmap_count value for the page passed to dma_map_page become 1 ?
> The caller have to make sure to complete dma_map_page before kunmap.


The caller doesn't have to call kmap() on pages it intends to use for 
DMA.

> Do I miss something ?

See above.

> > > As far as I understand, To make irq_disable to prevent this problem is 
> > > rather big cost.
> > 
> > How big?  Could you please elaborate on the significance of this cost?
> 
> I don't have a number. It depends on you for submitting this patch. 

My assertion is that the cost is negligible.  This is why I'm asking you 
why you think this is a big cost.


Nicolas
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