Re: phys-to-virt kernel mapping and ioremap()

[Jeff Garzik <jgarzik@mandrakesoft.mandrakesoft.com>]

On Thu, 20 Jul 2000, Timur Tabi wrote:

> I'm studying the code for __ioremap and I'm confused by something.
> 
> The phys_to_virt and virt_to_phys macros are very simple.  Basically, in kernel
> space, the virtual address is an offset of the physical address, so it's very
> simple.
> 
> __ioremap is supposed to take high PCI memory and map it to kernel space. 
> However, __ioremap() calls get_vm_area() which then calls kmalloc(), which
> allocates some memory from the heap.  Then remap_area_pages() is called, and
> that uses the three-level page tables to map the memory allocated by kmalloc to
> the PCI memory.
> 
> And that's where I'm confused.  Particularly:
> 
> 1) Doesn't this mapping break the phys_to_virt and virt_to_phys macros?
> 
> 2) kmalloc takes real physical memory from the kernel heap.  But then the
> virtual addresses are remapped to other physical memory.  What happens to the
> physical memory that kmalloc allocated?  Why isn't it freed?	

You are being confused by the allocation.  kmalloc only allocates a
vm_area_struct, which holds information about the new mapping.

	Jeff




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