tailieunhanh - LINUX DEVICE DRIVERS 3rd edition phần 8

nó không bao giờ được thông báo của phần mở rộng này, và bản đồ khu vực bổ sung cho trang không. Là một nhà văn người lái xe, bạn cũng có thể muốn ngăn chặn điều này loại hành vi, lập bản đồ các trang không vào cuối khu vực của bạn không phải là một điều rõ ràng xấu để làm, nhưng nó là chắc chắn rằng các lập trình viên muốn điều đó xảy ra. | Note that the user process can always use mremap to extend its mapping possibly past the end of the physical device area. If your driver fails to define a nopage method it is never notified of this extension and the additional area maps to the zero page. As a driver writer you may well want to prevent this sort of behavior mapping the zero page onto the end of your region is not an explicitly bad thing to do but it is highly unlikely that the programmer wanted that to happen. The simplest way to prevent extension of the mapping is to implement a simple nopage method that always causes a bus signal to be sent to the faulting process. Such a method would look like this struct page simple_nopage struct vm_area_struct vma unsigned long address int type return NOPAGE_SIGBUS send a SIGBUS As we have seen the nopage method is called only when the process dereferences an address that is within a known VMA but for which there is currently no valid page table entry. If we have used remap_pfn_range to map the entire device region the nopage method shown here is called only for references outside of that region. Thus it can safely return NOPAGE_SIGBUS to signal an error. Of course a more thorough implementation of nopage could check to see whether the faulting address is within the device area and perform the remapping if that is the case. Once again however nopage does not work with PCI memory areas so extension of PCI mappings is not possible. Remapping RAM An interesting limitation of remap_pfn_range is that it gives access only to reserved pages and physical addresses above the top of physical memory. In Linux a page of physical addresses is marked as reserved in the memory map to indicate that it is not available for memory management. On the PC for example the range between 640 KB and 1 MB is marked as reserved as are the pages that host the kernel code itself. Reserved pages are locked in memory and are the only ones that can be safely mapped to user space this .