get_user_pages函数详解
最近在看内核源码过程中，需要根据进程的虚拟地址，获取page结构体,经过资料查阅，发现了get_user_pages这个函数。1
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67/*
* get_user_pages() - pin user pages in memory
* @tsk: the task_struct to use for page fault accounting, or
* NULL if faults are not to be recorded.
* @mm: mm_struct of target mm
* @start: starting user address
* @nr_pages: number of pages from start to pin
* @write: whether pages will be written to by the caller
* @force: whether to force write access even if user mapping is
* readonly. This will result in the page being COWed even
* in MAP_SHARED mappings. You do not want this.
* @pages: array that receives pointers to the pages pinned.
* Should be at least nr_pages long. Or NULL, if caller
* only intends to ensure the pages are faulted in.
* @vmas: array of pointers to vmas corresponding to each page.
* Or NULL if the caller does not require them.
*
* Returns number of pages pinned. This may be fewer than the number
* requested. If nr_pages is 0 or negative, returns 0. If no pages
* were pinned, returns -errno. Each page returned must be released
* with a put_page() call when it is finished with. vmas will only
* remain valid while mmap_sem is held.
*
* Must be called with mmap_sem held for read or write.
*
* get_user_pages walks a process's page tables and takes a reference to
* each struct page that each user address corresponds to at a given
* instant. That is, it takes the page that would be accessed if a user
* thread accesses the given user virtual address at that instant.
*
* This does not guarantee that the page exists in the user mappings when
* get_user_pages returns, and there may even be a completely different
* page there in some cases (eg. if mmapped pagecache has been invalidated
* and subsequently re faulted). However it does guarantee that the page
* won't be freed completely. And mostly callers simply care that the page
* contains data that was valid *at some point in time*. Typically, an IO
* or similar operation cannot guarantee anything stronger anyway because
* locks can't be held over the syscall boundary.
*
* If write=0, the page must not be written to. If the page is written to,
* set_page_dirty (or set_page_dirty_lock, as appropriate) must be called
* after the page is finished with, and before put_page is called.
*
* get_user_pages is typically used for fewer-copy IO operations, to get a
* handle on the memory by some means other than accesses via the user virtual
* addresses. The pages may be submitted for DMA to devices or accessed via
* their kernel linear mapping (via the kmap APIs). Care should be taken to
* use the correct cache flushing APIs.
*
* See also get_user_pages_fast, for performance critical applications.
*/
long get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
unsigned long start, unsigned long nr_pages, int write,
int force, struct page **pages, struct vm_area_struct **vmas)
{
int flags = FOLL_TOUCH;
if (pages)
flags |= FOLL_GET;
if (write)
flags |= FOLL_WRITE;
if (force)
flags |= FOLL_FORCE;
return __get_user_pages(tsk, mm, start, nr_pages, flags, pages, vmas,
NULL);
}
函数各参数的详细说明在代码注释中已经写的很清楚，下面给出一个demo来看看这个函数的具体用法吧(建议读者先看下linux kernel模块化编程入门这篇文章)。
get_user_pages()能获取用户区进程使用内存的某个页(struct page)，然后可以在内核区通过kmap_atomic(), kmap()等函数映射到内核区线性地址，从而可以在内核区向其写入数据。
内核程序
1 |
|
Makefile文件
1 | obj-m += sample.o |
测试程序1
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int
main()
{
int fd;
char *ptr;
fd = open("/dev/Sample", O_RDWR);
if (fd < 0) {
perror("error");
}
posix_memalign((void **)&ptr, 4096, 4096);
memcpy(ptr, "krishna", strlen("krishna")); //Write String to Driver
write(fd, ptr, 4096);
printf("data is %s\n", ptr); //Read Data from Driver
close(fd);
}
参考资料：