Hi all,
I am lost figuring out how this piece of code works (from Linux Device
Driver 3ed, page 428).
struct page *simple_vma_nopage(struct vm_area_struct *vma,
unsigned long address, int *type)
{
struct page *pageptr;
unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
unsigned long physaddr = address - vma->vm_start + offset;
unsigned long pageframe = physaddr >> PAGE_SHIFT;
if (!pfn_valid(pageframe))
return NOPAGE_SIGBUS;
pageptr = pfn_to_page(pageframe);
get_page(pageptr);
if (type)
*type = VM_FAULT_MINOR;
return pageptr;
}
My doubts are,
how does the statements,
offset = vma->vm_pgoff << PAGE_SHIFT;
physaddr = address - vma->vm_start + offset;
hold true?
AFAIK, from mm.h, vma->vm_pgoff is the Offset (within vm_file) in
PAGE_SIZE units, *not* PAGE_CACHE_SIZE. That gives variable offset =
offset within file in bytes.
address is the virtual address whose page frame is requested.
Then, physaddr = address (virtual address) - vma->vm_start (virtual
address of start of the vma) + offset within the file in bytes.
How can this be true? What am I missing?
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