mmap for Linux drivers
June 26, 2009
Due to an assignment for the Operating Systems Lab, which I’m attending this semester, I started reading about character device drivers for the Linux Kernel. We were a given an incomplete driver, which handled the communication with the hardware, as well as some other issues, and we had to implement the ‘upper layer’ of the driver, which did the communication with the userspace, and handled some locking issues.
Linux Device Drivers(LDD) was very helpful to start with(trying to read kernel code for the first time can be a terrifying experience). However, when I got to the point, where I had to implement the mmap method for the driver, LDD was a bit dated. After some googling and with the help of the Linux Cross Reference, I found out the changes in the Linux Kernel API, and sucessfully implemented mmap(or so I hope :P).
Due to a number of reasons, the nopage method, as well as the populate and the nopfn methods, have been completely removed from the vm_opeartions struct, and the new fault method is used instead for handling page faults.
Besides the changes in the fault handlers, recent kernel releases added another method to map pages to userspace(remap_pfn_range is often used for that purpose). The method is called vm_insert_page, which allows the driver to map a single page to userspace, and can be very useful when you need to map just one page-aligned buffer, which was allocated inside the driver, to userspace.
So, replacing nopage with fault, and using vm_insert_page (simpler code and better suited to the needs of the driver I was writing) instead of remap_pfn_range, did the job.
Btw, in LDD Ch15, it states that it’s not possible to map conventional RAM with remap_pfn_range(ie pages you get from get_free_page) because remap_pfn_range only maps pages with the PG_reserved flag set. However, drivers would manually set the PG_reserved flag to make remap_pfn_range work, although the proper way of remapping ordinary RAM was with the nopage method(described in LDD). Tweaking the PG_reserved flag was considered bad practice, and so Linux 2.6.15 practically removed the PG_reserved flag. A couple of changes were made to the VMA flags as well, including the new vm_insert_page method, and it was made possible to map ‘ordinary’ RAM with remap_pfn_range. Since my knowledge of the memory management in Linux and the VM subsystem is minimal, I can’t explain much more(maybe in another post after a few months ; ).