[lkml]   [2006]   [Oct]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: mmaping a kernel buffer to user space
Hi Jiri,

The fact that it does not works with RAM is well documented in LDD3,
pages 430++. It says (and I tested) that remap_xxx_range does not work
in this case. They suggest a method using nopage, similar to the one I

I do not see why remap_xxx_range has the limitation, but it is there.
The question is then: can the limitation be removed, or can we implement
a new function that maps RAM all at once without the need for a nopage

In any case, here is the code.

Best Wishes,


To allocate (inside an IOctl cmd):

retptr = pci_alloc_consistent( privdata->pdev, kmem_handle->size,
&(kmem_entry->dma_handle) );
if (retptr == NULL)
goto kmem_alloc_mem_fail;
kmem_entry->cpua = (unsigned long)retptr;
kmem_entry->size = kmem_handle->size;
kmem_entry->id = atomic_inc_return(&privdata->kmem_count) - 1;

To mmap (inside mmap fops, this DOES NOT works):

/* Map the Buffer to the VMA */
ret = remap_pfn_range(
__pa(kmem_entry->cpua) >> PAGE_SHIFT,
vma->vm_page_prot );
ret = remap_page_range(
vma->vm_page_prot );

Forcing me to this:

(in mmap):

/* Set VM operations */
vma->vm_private_data = kmem_entry;
vma->vm_ops = &pcidriver_vm_operations;

(and the nopage vm_ops):

/* Maps physical memory to user space */
struct page *pcidriver_vm_nopage(struct vm_area_struct *vma, unsigned
long address, int * type) {

pcidriver_kmem_entry_t *kmem_entry;
struct page *page = NOPAGE_SIGBUS;
unsigned long pfn_offset,pfn;

/* Get private data for the page */
kmem_entry = vma->vm_private_data;

/* All checks where done during the mmap_kmem call, so we can safely
* just map the offset of the vm area to the offset of the region,
* that is guaranteed to be contiguos */

pfn_offset = (address) - (vma->vm_start);
pfn = (__pa(kmem_entry->cpua) + pfn_offset ) >> PAGE_SHIFT;

if (!pfn_valid(pfn)) {
mod_info("Invalid pfn in nopage() - 0x%lx \n", pfn);

page = pfn_to_page( pfn );

if (type)

return page;


Jiri Slaby wrote:
> 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,
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2006-10-31 17:29    [W:0.174 / U:13.432 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site