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Subject: Re: How to efficiently handle DMA and cache on ARMv7 ? (was
 "Is get_user_pages() enough to prevent pages from being swapped out ?")
From: "David Xiao" <dxiao@broadcom.com>
To: "Russell King - ARM Linux" <linux@arm.linux.org.uk>
cc: "Laurent Pinchart" <laurent.pinchart@ideasonboard.com>,
       "Robin Holt" <holt@sgi.com>,
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Date: Fri, 7 Aug 2009 15:25:52 -0700
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On Fri, 2009-08-07 at 13:28 -0700, Russell King - ARM Linux wrote:

> The kernel direct mapping maps all system (low) memory with normal
> memory cacheable attributes.
> 
> So using vmalloc, dma_alloc_coherent, using pages in userspace all
> create duplicate mappings of pages.
> 

If we do want to remove all these duplicate mappings, as part of
solution to deal with the speculative prefetching, probably one way is
to not map all the RAM into the direct-mapped space at paging_init()
time, and instead map them on-demand by different upper layer allocation
functions, such as vmalloc/dma_alloc_coherent/do_brk/kmalloc/
get_free_pages/etc. But then the distinction between upper layer
allocation functions and non-upper layer ones must be made clear though.

I know that mapping the RAM at paging_init() time can take advantage of
1M section mapping most of the time, and thus save many 1KB L2 page
tables. But a lot of memory still ends up being remapped with L2 page
tables later on, and meanwhile 1KB might not be as "precious" as it used
to be as well-:)

David