Re: [PATCH v3 5/8] mm: Device exclusive memory access

On Fri, Feb 26, 2021 at 06:18:29PM +1100, Alistair Popple wrote:

+/**

• • make_device_exclusive_range() - Mark a range for exclusive use by a device
• • @mm: mm_struct of assoicated target process
• • @start: start of the region to mark for exclusive device access
• • @end: end address of region
• • @pages: returns the pages which were successfully mark for exclusive acces
• • Returns: number of pages successfully marked for exclusive access
• • This function finds the ptes mapping page(s) to the given address range and
• • replaces them with special swap entries preventing userspace CPU access. On
• • fault these entries are replaced with the original mapping after calling MMU
• • notifiers.
• */

+int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,

• 		unsigned long end, struct page **pages)
  

+{

• long npages = (end - start) >> PAGE_SHIFT;
• long i;
• npages = get_user_pages_remote(mm, start, npages,

• 		       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
  

• 		       pages, NULL, NULL);
  

• for (i = 0; i < npages; i++) {

• if (!trylock_page(pages[i])) {
  

• 	put_page(pages[i]);
  

• 	pages[i] = NULL;
  

• 	continue;
  

• }
  

• if (!try_to_protect(pages[i])) {
  

Isn't this racy? get_user_pages returns the ptes at an instant in time, they could have already been changed to something else?

I would think you'd want to switch to the swap entry atomically under th PTLs?

Jason