Re: [PATCH v7 5/8] mm: Device exclusive memory access

On Fri, Mar 26, 2021 at 11:08:02AM +1100, Alistair Popple wrote:

diff --git a/mm/memory.c b/mm/memory.c index 3a5705cfc891..33d11527ef77 100644 +++ b/mm/memory.c @@ -781,6 +781,27 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, pte = pte_swp_mkuffd_wp(pte); set_pte_at(src_mm, addr, src_pte, pte); }

• } else if (is_device_exclusive_entry(entry)) {

• page = pfn_swap_entry_to_page(entry);
  

• get_page(page);
  

• rss[mm_counter(page)]++;
  

• if (is_writable_device_exclusive_entry(entry) &&
  

•     is_cow_mapping(vm_flags)) {
  

• 	/*
  

• 	 * COW mappings require pages in both
  

• 	 * parent and child to be set to read.
  

• 	 */
  

• 	entry = make_readable_device_exclusive_entry(
  

• 					swp_offset(entry));
  

• 	pte = swp_entry_to_pte(entry);
  

• 	if (pte_swp_soft_dirty(*src_pte))
  

• 		pte = pte_swp_mksoft_dirty(pte);
  

• 	if (pte_swp_uffd_wp(*src_pte))
  

• 		pte = pte_swp_mkuffd_wp(pte);
  

• 	set_pte_at(src_mm, addr, src_pte, pte);
  

• }
  

This needs to have the same logic as we now have in copy_present_page(). The page *is* present and we can't copy the PTE value hidden in a swap entry if we can't copy the PTE normally.

The code should be shared because nobody is going to remember about this corner case.

Jason