Re: returning non-ram via ->nopage, was Re: [patch] mspec driver for 2.6.12-rc2-mm3

[Russell King]

On Wed, Apr 27, 2005 at 02:03:50PM -0400, Jes Sorensen wrote:
> >>>>> "Christoph" == Christoph Hellwig <hch@xxxxxxxxxxxxx> writes:
> 
> Christoph> On Wed, Apr 27, 2005 at 11:53:15AM -0400, Jeff Garzik
> Christoph> wrote:
> >> I don't see anything wrong with a ->nopage approach.
> >> 
> >> At Linus's suggestion, I used ->nopage in the implementation of
> >> sound/oss/via82cxxx_audio.c.
> 
> Christoph> The difference is that you return kernel memory (actually
> Christoph> pci_alloc_consistant memory that has it's own set of
> Christoph> problems), while this is memory not in mem_map, so he
> Christoph> allocates some regularly kernel memory too to have a struct
> Christoph> page and just leaks it
> 
> Are you suggesting then that we change do_no_page to handle this as a
> special return value then?

If you're looking to mmap dma memory, ARM already supports the API
which was discussed (although not properly imho) on linux-arch.
I previously posted a potential patch for x86, but it has the
problem that remap_pfn_range() will not work on such memory because
it isn't marked reserved.

In addition, if you're mmaping dma memory on x86 as is, you're
providing a potential security hole - the x86 DMA memory allocator
does not extend its zeroing to cover the entire last page of the
allocation.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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