Re: mmaping a kernel buffer to user space
- From: Guillermo Marcus <marcus@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 31 Oct 2006 17:25:54 +0100
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
implement.
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
implementation?
In any case, here is the code.
Best Wishes,
Guillermo
/*************************************************/
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 */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,10)
ret = remap_pfn_range(
vma,
vma->vm_start,
__pa(kmem_entry->cpua) >> PAGE_SHIFT,
kmem_entry->size,
vma->vm_page_prot );
#else
ret = remap_page_range(
vma,
vma->vm_start,
__pa(kmem_entry->cpua),
kmem_entry->size,
vma->vm_page_prot );
#endif
...
Forcing me to this:
(in mmap):
...
/* Set VM operations */
vma->vm_private_data = kmem_entry;
vma->vm_ops = &pcidriver_vm_operations;
pcidriver_vm_open(vma);
...
(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);
return NOPAGE_SIGBUS;
}
page = pfn_to_page( pfn );
get_page(page);
if (type)
*type = VM_FAULT_MINOR;
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,
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