On Thu, Jul 14, 2016 at 09:04:18PM -0400, Rik van Riel wrote:

On Fri, 2016-07-15 at 09:20 +1000, Balbir Singh wrote:

quoted

quoted

==
+		   ((unsigned long)end & (unsigned
long)PAGE_MASK)))
+		return NULL;
+
+	/* Allow if start and end are inside the same compound
page. */
+	endpage = virt_to_head_page(end);
+	if (likely(endpage == page))
+		return NULL;
+
+	/* Allow special areas, device memory, and sometimes
kernel data. */
+	if (PageReserved(page) && PageReserved(endpage))
+		return NULL;

If we came here, it's likely that endpage > page, do we need to check
that only the first and last pages are reserved? What about the ones
in
the middle?

I think this will be so rare, we can get away with just
checking the beginning and the end.

But do we want to leave a hole where an aware user space
can try a longer copy_* to avoid this check? If it is unlikely
should we just bite the bullet and do the check for the entire
range?

Balbir Singh.