Mmap and remap_page_range(), in the nutshell.

心已赠人 2022-08-24 04:16 18阅读 0赞

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           <p style="margin-top: 10px; margin-bottom: 10px; ">繼 Linux programming 課程紀錄「</p> 
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[小談 mmap() 與 VMA][mmap_ _ VMA]」後，今天在 Linux device driver 課程再聊到有關 VMA 與 mmap driver function 的重要觀念；重點紀錄如下。

當 user-space 呼叫 mmap() system call wrapper function 後，kernel 會在 process address space 裡建立新的 VMA，並在 callback mmap driver function 時將「該」VMA 傳遞給我們的驅動程式。

因此，在驅動程式裡，只需要利用 remap\_page\_range() 將 kernel-space 的 memory：

> \- I/O memory
> 
> \- RAM（reserved pages）
> 
> \- Virtual address space（reserved pages）

對應到該 VMA 即可。最後，寫出了以下的 skeleton code：

>     // refer to: bttv-dirver.c
>     int do_card_mmap(struct vm_area_struct *vma, char *adr, unsigned long size)
>     {
>         unsigned long start = (unsigned long)adr;
>         unsigned long page, pos;
>     
>         pos = (unsigned long)BaseIOAddress;
>     #if 1  // page or the total size ?
>         while (size >0) {
>            page = pos;
>            if (remap_page_range(vma, start, page, PAGE_SIZE, PAGE_SHARED))
>     	   return -EAGAIN;
>            start+=PAGE_SIZE;
>            pos+=PAGE_SIZE;
>            size-=PAGE_SIZE;
>         }
>     #else
>         if (remap_page_range(vma, start, pos, size, PAGE_SHARED))
>            return -EAGAIN;
>     #endif
>         return 0;
>     }
>     
>     // refer to: videodev.c
>     int card_mmap(struct file *filp, struct vm_area_struct *vma)
>     {
>        do_card_mmap(vma, (char *)vma->vm_start, (unsigned long)(vma->vm_end-vma->vm_start));
>        return 0;
>     }
>     /**************************************************/
>     
>     struct file_operations card_fops = {
>       open:card_open,
>       release:card_release,
>       ioctl:card_ioctl,
>       mmap:card_mmap,
>     };

remap\_page\_range() 的原理是去修改 page table，但是要不要以 "page" 為最小單位做 page table 的修改（呼叫 'remap\_page\_range'），必須視情況而定！

[mmap_ _ VMA]: http://www.jollen.org/blog/2007/03/mmap_vma.html