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Beginning Linux Programming Third Edition phần 10

Đây là phiên bản cao snipped của cấu trúc, bạn có thể tìm thấy nó trong linux / mm.h. Chúng tôi sẽ bao gồm các thành viên mà chúng tôi thực sự sẽ cần sau này. vm_start và vm_end hiện sự bắt đầu và kết thúc của khu vực bộ nhớ ảo, và vm_page_prot là bảo vệ | Device Drivers Virtual Memory Areas Above the page table reside the virtual memory areas. These constitute a map of contiguous virtual memory addresses as handed out to an application. struct vm_area_struct . unsigned long vm_start unsigned long vm_end . pgprot_t vm_page_prot . struct vm_operations_struct vm_ops unsigned long vm_pgoff struct file vm_file . This is a highly snipped version of the structure you can find it in linux mm.h. We will cover the members that we will actually need later on. vm_start and vm_end represent the beginning and end of the virtual memory area and vm_page_prot is the protection attributes assigned to it whether it is shared private executable and so forth. vm_ops is similar to the file_operations structure used with character and block devices and forms an analogous abstraction to operation on virtual memory areas. Finally vm_offset is the offset into the area and vm_file is used in correlation with memory mapping of files. We ll take a much closer look at this structure when we dissect the mmap functions of schar. The mappings made by a specific process can be seen in proc PID maps. Each one corresponds to a separate vm_area_struct and the size span and protection associated with the mapping can be read from the proc entry among other things. Address Space The entire addressable area of memory 4 GB on 32-bit platforms is split into two major areas kernel space and user or application space. PAGE_OFFSET defines this split and is actually configurable in asm page.h. The kernel space is located above the offset and user space is kept below. The default for PAGE_OFFSET on the Intel platform is 0xc0000000 and thus provides the kernel with approximately 1 GB of memory leaving 3 GB for user space consumption. On the Intel platform the virtual addresses seen from the kernel are therefore a direct offset from the physical address. This isn t always the case and primitives to convert between the two must thus be used. See Figure 18-4 for a .