userfaultfd: wp: support swap and page migration

For either swap and page migration, we all use the bit 2 of the entry to
identify whether this entry is uffd write-protected.  It plays a similar
role as the existing soft dirty bit in swap entries but only for keeping
the uffd-wp tracking for a specific PTE/PMD.

Something special here is that when we want to recover the uffd-wp bit
from a swap/migration entry to the PTE bit we'll also need to take care of
the _PAGE_RW bit and make sure it's cleared, otherwise even with the
_PAGE_UFFD_WP bit we can't trap it at all.

In change_pte_range() we do nothing for uffd if the PTE is a swap entry.
That can lead to data mismatch if the page that we are going to write
protect is swapped out when sending the UFFDIO_WRITEPROTECT.  This patch
also applies/removes the uffd-wp bit even for the swap entries.

Signed-off-by: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Bobby Powers <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Denis Plotnikov <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Kirill A . Shutemov" <[email protected]>
Cc: Martin Cracauer <[email protected]>
Cc: Marty McFadden <[email protected]>
Cc: Maya Gokhale <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Shaohua Li <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 877fd239b6ff..9a6f06de183b 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -68,6 +68,8 @@ static inline swp_entry_t pte_to_swp_entry(pte_t pte)
 
 	if (pte_swp_soft_dirty(pte))
 		pte = pte_swp_clear_soft_dirty(pte);
+	if (pte_swp_uffd_wp(pte))
+		pte = pte_swp_clear_uffd_wp(pte);
 	arch_entry = __pte_to_swp_entry(pte);
 	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
 }