Memory Mapping Buffers smaller than page size?

Memory Mapping Buffers smaller than page size?

[Ravi Gupta]

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Hi,

I am new to linux device drivers development. I have created 16 buffers of
size 256 bytes each(using kmalloc()) in my device driver code. I want to
memory map these buffers to user space. Now is it possible to memory map
these buffer(16*256 = 4096 = 1 page on 32 bit linux) into a single page in
user space OR i have to map them in individual pages in user space? Note,
all the buffers may not be stored in continuous memory location.

Thanks in advance
Ravi Gupta

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Re: Memory Mapping Buffers smaller than page size?

[Simon Richter]

Hi,

On Wed, Jul 28, 2010 at 06:44:10PM +0530, Ravi Gupta wrote:

> I am new to linux device drivers development. I have created 16 buffers of
> size 256 bytes each(using kmalloc()) in my device driver code. I want to
> memory map these buffers to user space. Now is it possible to memory map
> these buffer(16*256 = 4096 = 1 page on 32 bit linux) into a single page in
> user space OR i have to map them in individual pages in user space? Note,
> all the buffers may not be stored in continuous memory location.

Pages are the smallest unit for mappings, so each buffer would end up in
its own mapping. If you want the buffers to be accessible without an
offset, then you cannot have them in continuous locations, as you cannot
map memory from the middle of a page to the beginning either.

So your options are: one page per buffer (wasteful, but gives you
granular access control), or allocating all the buffers as a single
block.

   Simon

Re: Memory Mapping Buffers smaller than page size?

[Ravi Gupta]

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Hi Simon,

Thanks for the quick reply. One more thing I want to ask is what if I create
a dma pool (using pci_pool_create()), allocate dma buffers from that pool
and then try to memory map them? will the buffers in that case will be
continuous and is it possible to memory map them in a single user space
page?

Thanks in advance
Ravi Gupta

On Wed, Jul 28, 2010 at 7:51 PM, Simon Richter <Simon.Richter@hogyros.de>wrote:

> Hi,
>
> On Wed, Jul 28, 2010 at 06:44:10PM +0530, Ravi Gupta wrote:
>
> > I am new to linux device drivers development. I have created 16 buffers
> of
> > size 256 bytes each(using kmalloc()) in my device driver code. I want to
> > memory map these buffers to user space. Now is it possible to memory map
> > these buffer(16*256 = 4096 = 1 page on 32 bit linux) into a single page
> in
> > user space OR i have to map them in individual pages in user space? Note,
> > all the buffers may not be stored in continuous memory location.
>
> Pages are the smallest unit for mappings, so each buffer would end up in
> its own mapping. If you want the buffers to be accessible without an
> offset, then you cannot have them in continuous locations, as you cannot
> map memory from the middle of a page to the beginning either.
>
> So your options are: one page per buffer (wasteful, but gives you
> granular access control), or allocating all the buffers as a single
> block.
>
>   Simon
>

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