looking for help with details of mapping pages to kernel and userspace

looking for help with details of mapping pages to kernel and userspace

[Chris Friesen]

I have a box with 1.5GB of memory and HIGHMEM enabled.

I would like to have a page of memory accessable from userspace, regular kernel 
space, and late asm code on return from exceptions with no valid C context.  As 
far as I can see, this implies that the page tables need to be in lowmem since 
having to do a kmap_atomic in asm would be painful.

It has been suggested that we could do something like the following, where this 
is running on behalf of userspace which passed in a desired mapping address:

mypage =  __get_free_page(GFP_KERNEL);
SetPageReserved(virt_to_page(mypage));
remap_page_range(vma->vm_start, virt_to_phys((void *) mypage), PAGE_SIZE, 
vma->vm_page_prot);

This seems to work, but I'm concerned about cleanup.  When the task dies, how 
much cleanup will be done on the page?  Does the memory subsystem handle 
everything, or do I need to explicitly do something like:

reservedClearPageReserved(virt_to_page(mypage));
free_page(mypage);

Is there a better way of doing this?  I had considered mmap(), but I think that 
could give me pages with their page tables up in high memory.


Thanks,

Chris