Specifications

This is the Title of the Book, eMatter Edition
Copyright © 2005 O’Reilly & Associates, Inc. All rights reserved.
The mmap Device Operation
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Most of the work is then performed by nopage. In the scullp implementation, the
address parameter to nopage is used to calculate an offset into the device; the offset is
then used to look up the correct page in the scullp memory tree:
struct page *scullp_vma_nopage(struct vm_area_struct *vma,
unsigned long address, int *type)
{
unsigned long offset;
struct scullp_dev *ptr, *dev = vma->vm_private_data;
struct page *page = NOPAGE_SIGBUS;
void *pageptr = NULL; /* default to "missing" */
down(&dev->sem);
offset = (address - vma->vm_start) + (vma->vm_pgoff << PAGE_SHIFT);
if (offset >= dev->size) goto out; /* out of range */
/*
* Now retrieve the scullp device from the list,then the page.
* If the device has holes, the process receives a SIGBUS when
* accessing the hole.
*/
offset >>= PAGE_SHIFT; /* offset is a number of pages */
for (ptr = dev; ptr && offset >= dev->qset;) {
ptr = ptr->next;
offset -= dev->qset;
}
if (ptr && ptr->data) pageptr = ptr->data[offset];
if (!pageptr) goto out; /* hole or end-of-file */
page = virt_to_page(pageptr);
/* got it, now increment the count */
get_page(page);
if (type)
*type = VM_FAULT_MINOR;
out:
up(&dev->sem);
return page;
}
scullp uses memory obtained with get_free_pages. That memory is addressed using
logical addresses, so all scullp_nopage has to do to get a
struct page pointer is to call
virt_to_page.
The scullp device now works as expected, as you can see in this sample output from
the mapper utility. Here, we send a directory listing of /dev (which is long) to the
scullp device and then use the mapper utility to look at pieces of that listing with
mmap:
morgana% ls -l /dev > /dev/scullp
morgana% ./mapper /dev/scullp 0 140
mapped "/dev/scullp" from 0 (0x00000000) to 140 (0x0000008c)
total 232
crw------- 1 root root 10, 10 Sep 15 07:40 adbmouse
,ch15.13676 Page 433 Friday, January 21, 2005 11:04 AM