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

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Message-ID: <454899E9.1090900@ti.uni-mannheim.de>
Date:	Wed, 01 Nov 2006 13:58:17 +0100
From:	Guillermo Marcus Martinez <marcus@...uni-mannheim.de>
To:	linux-kernel@...r.kernel.org
Subject: Re: mmaping a kernel buffer to user space

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().

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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