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

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Date:	Tue, 31 Oct 2006 17:00:59 +0100
From:	Jiri Slaby <jirislaby@...il.com>
To:	Guillermo Marcus <marcus@...uni-mannheim.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: mmaping a kernel buffer to user space

Guillermo Marcus wrote:
> Hi all,
> 
> I recently run with the following situation while developing a PCI
> driver. The driver allocates memory for a PCI device using
> pci_alloc_consistent as this memory is going to be used to perform DMA
> transfers. To pass the data from/to the user application, I mmap the
> buffer into userspace. However, if I try to use remap_pfn_range
> (>=2.6.10) or the older remap_page_range(<=2.6.9) for mmaping, it ends
> up creating a new buffer, because they do not support RAM mapping, then
> pagefaulting to the VMA and by default allocating new pages. Therefore,
> I had to implement the nopage method and mmap one page at a time as they
> fault.
> 
> However, to my point of view, this is unnecessary. The memory is already
> allocated, the memory is locked because it is consistent, and it may be
> a (very small) performance and stability issue to do them one-by-one.
> Why can't I simply mmap it all at once? am I missing some function? More
> important, why can't remap_{pfn/page}_range handle it?

Piece of code please. pci_alloc_consistent calls __get_free_pages, and there
should be no problem with mmaping this area.

regards,
-- 
http://www.fi.muni.cz/~xslaby/            Jiri Slaby
faculty of informatics, masaryk university, brno, cz
e-mail: jirislaby gmail com, gpg pubkey fingerprint:
B674 9967 0407 CE62 ACC8  22A0 32CC 55C3 39D4 7A7E
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