linux-kernel - Re: mmaping a kernel buffer to user space

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Message-ID: <4548B7EC.7060006@ti.uni-mannheim.de>
Date:	Wed, 01 Nov 2006 16:06:20 +0100
From:	Guillermo Marcus Martinez <marcus@...uni-mannheim.de>
To:	linux-kernel@...r.kernel.org
Subject: Re: mmaping a kernel buffer to user space

yogeshwar sonawane schrieb:
> On 11/1/06, Guillermo Marcus Martinez <marcus@...uni-mannheim.de> wrote:
>> Rolf Offermanns schrieb:
>> > Guillermo Marcus <marcus <at> ti.uni-mannheim.de> writes:
>> >> Note: I am using kernel 2.6.9 for these tests, as it is required by my
>> >> current setup. Maybe this issue has already been addressed in newer
>> >> kernel. If that is the case, please let me know.
>> >
>> > Have a look at this article:
>> >
>> > "The evolution of driver page remapping"
>> > http://lwn.net/Articles/162860/
>> >
>> > It should make things clearer.
>> >
>> > The "API changes in the 2.6 kernel series" page is also a very good
>> read:
>> > http://lwn.net/Articles/2.6-kernel-api/
>> >
>> > HTH,
>> > Rolf
>>
>> Thanks for the links!
>>
>> Yes, it looks like a step in the right direction. However, the article
>> says about vm_insert_page(): "...What it does require is that the page
>> be an order-zero allocation obtained for this purpose...", therefore
>> making it also unusable for this case (mmaping a pci_alloc_consistent).
>>
>> I think the limitation (being order zero), is related to the page
>> counting, as I understand that for bigger order allocations, only the
>> first-page counter is incremented (not every page). If that is a
>> problem, I guess I would also see a problem with my workaround, and I
>> see none (yet). So I may try in a newer kernel and see if I can use it
>> to walk the pages on the mmap without using the nopage().
> 
> Setting 'PG_reserved' bit of all allocated pages & then calling
> remap_page/pfn_range()
> will do the things for 2.6.9.
> 

I will give it a try. I guess it may not be equivalent to setting
VM_RESERVED before calling remap_page/pfn_range(). Is this platform
specific, or is intended behavior/usage of remap_page/pfn_range()?


>>
>> My suggestion would be to add two functions: pci_map_consistent() and
>> dma_map_coherent() to address this issue, and their corresponding
>> unmap's. That will make sure all that is needed is done, is a clean and
>> consistent with the pci_ and dma_ APIs, and fills a mmap requirement not
>> covered by the other functions.
>>
>> Best wishes,
>> Guillermo
>>
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