Howdy, I'm trying to get a better handle on how the kernel handles addresses. Could anybody who is so inclined have a look at the following and tell me (gently) where I've gone wrong? There are several different ways of specifying an address in the kernel, and the code is not always all that explicit on which it's using in any given place. The terminology doesn't even seem to be there to make the distinction, so I've made up some of my own. Here's the various ways I've been able to find to refer to a page: struct page The cookie used, increasingly, to represent any page in the system. Also where much of the page's housekeeping information lives. page frame number Essentially an index into mem_map - what you would once get from MAP_NR. There is very little use of page frame numbers in the 2.4 kernel - struct page is used instead. physical address An address as known by the low-level hardware. In the modern world, these can be 64-bit quantities, even on 32-bit systems. These are the addresses used by /dev/mem - which appears to work only for low memory. direct-mapped kernel virtual address An address as seen by the kernel - on many architectures it's just the physical address with an offset applied. Much of the kernel treats these as if they were physical addresses, but they are virtual and do not work for highmem pages. Back in my VMS days we would have called these "logical" addresses. There appears, however, to be a form of Godwin's law that applies to this list: if somebody mentions VMS the conversation stops. So I won't do that. pure kernel virtual address An address which can be dereferenced by the kernel, but which does not directly map to a physical address. Examples include addresses returned by vmalloc() and kmap(). The union of the direct-mapped and pure kernel virtual addresses is exported by /dev/kmem. user virtual address A basic virtual address used in user space. There are ways of moving between various types of addresses: virt_to_page() Maps a "logical" kernel address to a struct page. Does not work for pure kernel virtual or user virtual addresses. __pa() Turns a logical kernel address into a physical address. Also known as virt_to_phys(). __va() Turns a (lowmem) physical address into a logical address. Also known as phys_to_virt(). page_address() Maps a struct page to a logical (for lowmem) or kernel virtual (for highmem which has been mapped) address. The comment on the i386 version claims it returns the "permanent" address, but that is not true for highmem pages which have been kmap'd. pte_page() Turns a page table entry into a struct page. There does not appear to be a function to go from a user or kernel virtual address to a physical address or struct page - one must manually walk through the levels of page tables to get there via the pte. Any corrections / suggestions would be much appreciated. jon Jonathan Corbet Executive editor, LWN.net corbet@lwn.net - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/