This is the first time I have seen the word "unity-mapping"......if it is your invention....congratulations!!!! U have created a new term. Usually I heard of "identity mapping", "direct mapping", linear mapping, vs "non-linear mapping". The formula is basically equivalent to __pa(), and all the different variation: __va(), virt_to_page(), pfn_to_kaddr() etc. It is all just using a straightforward formula. This is because virtual address and physical addr are inter-convertible directly in the ZONE_NORMAL range. But not in the ZONE_HIGH area. kmalloc() always returned addresses in these range. All the confusing API like pud*, pgd*, pmd*() API also hinges on this characteristic to convert directly between physical and virtual, or to extract out page frame number, PTE etc from the virtual/physical addresses. When u learn identity mapping, it really helped to know versus what? Ie, what are the other mapping available? And this is where vmalloc() comes in. It returned addresses in the ZONE_HIGH area, where kernel modules are loaded as well. The mappings are kept in the task-specific VMA linked list (list head is at current->mm) or anonymous memory's head (at anon_vma->head). Conversion between physical and virtual is a bit more difficult, where u really have to walk through the VMA lists or the reverse mapping lists to search for answer. Other variations (quoted from a book): Persistent mappings are used to map non-persistent pages from the highmem area into the kernel. Fixmaps are virtual address space entries associated with a fixed but freely selectable page in physical address space. In contrast to directly mapped pages that are associated with RAM memory by means of a fixed formula, the association between a virtual fixmap address and the position in RAM memory can be freely defined and is then always observed by the kernel. Current work: vmap() reworking: http://lwn.net/Articles/304188/ have fun!!!! Yours Confusingly, Peter Teoh On Mon, Mar 2, 2009 at 9:42 AM, NAHieu <nahieu@xxxxxxxxx> wrote: > Sorry that I accientally pushed the button before completing email. So > the full message is like below. > > Hi, > > In Linux kernel, we have unity-mapping feature, which maps the virtual > addresses in kernel area (above PAGE_OFFSET) to physical addresses in > the format: > y = x - PAGE_OFFSET > > in which "y" is physical address corresponding to the virtual address "x" > > I understand that this unity-mapping is correct with all the virtual > addresses in the range of [PAGE_OFFSET, ZONE-NORMAL\ area. > > Is that assumption correct? > > Thanks a lot, > H > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx > Please read the FAQ at http://kernelnewbies.org/FAQ > > -- Regards, Peter Teoh -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
