On Fri, May 29, 2020 at 04:43:08PM -0700, John Hubbard wrote:

+CASE 5: Pinning in order to write to the data within the page
+-------------------------------------------------------------
+Even though neither DMA nor Direct IO is involved, just a simple case of "pin,
+access page's data, unpin" can cause a problem. Case 5 may be considered a
+superset of Case 1, plus Case 2, plus anything that invokes that pattern. In
+other words, if the code is neither Case 1 nor Case 2, it may still require
+FOLL_PIN, for patterns like this:
+
+Correct (uses FOLL_PIN calls):
+    pin_user_pages()
+    access the data within the pages
+    set_page_dirty_lock()
+    unpin_user_pages()
+
+INCORRECT (uses FOLL_GET calls):
+    get_user_pages()
+    access the data within the pages
+    set_page_dirty_lock()
+    put_page()

Why does this case need to pin?  Why can't it just do ...

	get_user_pages()
	lock_page(page);
	... modify the data ...
	set_page_dirty(page);
	unlock_page(page);