mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

For VMAs that don't want write notifications, PTEs created for read faults
have their write bit set.  If the read fault happens after VM_SOFTDIRTY is
cleared, then the PTE's softdirty bit will remain clear after subsequent
writes.

Here's a simple code snippet to demonstrate the bug:

  char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
                 MAP_ANONYMOUS | MAP_SHARED, -1, 0);
  system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */
  assert(*m == '\0');     /* new PTE allows write access */
  assert(!soft_dirty(x));
  *m = 'x';               /* should dirty the page */
  assert(soft_dirty(x));  /* fails */

With this patch, write notifications are enabled when VM_SOFTDIRTY is
cleared.  Furthermore, to avoid unnecessary faults, write notifications
are disabled when VM_SOFTDIRTY is set.

As a side effect of enabling and disabling write notifications with
care, this patch fixes a bug in mprotect where vm_page_prot bits set by
drivers were zapped on mprotect.  An analogous bug was fixed in mmap by
commit c9d0bf241451 ("mm: uncached vma support with writenotify").

Signed-off-by: Peter Feiner <[email protected]>
Reported-by: Peter Feiner <[email protected]>
Suggested-by: Kirill A. Shutemov <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Jamie Liu <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>