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    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);
    return NOPAGE_SIGBUS;

    page = pfn_to_page( pfn );

    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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      Last update: 2006-10-31 17:29    [W:0.024 / U:22.728 seconds]
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