mm: increase the default mlock limit from 32k to 64k

By default, non-privileged tasks can only mlock() a small amount of
memory to avoid a DoS attack by ordinary users. The Linux kernel
defaulted to 32k (on a 4k page size system) to accommodate the needs of
gpg.

However, newer gpg2 needs 64k in various circumstances and otherwise
fails miserably, see bnc#329675.

Change the default to 64k, and make it more agnostic to PAGE_SIZE.

Signed-off-by: Kurt Garloff <[e-mail]>
Signed-off-by: Nick Piggin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/resource.h

···
 #define _STK_LIM	(8*1024*1024)
 
 /*
- * GPG wants 32kB of mlocked memory, to make sure pass phrases
+ * GPG2 wants 64kB of mlocked memory, to make sure pass phrases
  * and other sensitive information are never written to disk.
  */
-#define MLOCK_LIMIT	(8 * PAGE_SIZE)
+#define MLOCK_LIMIT	((PAGE_SIZE > 64*1024) ? PAGE_SIZE : 64*1024)
 
 /*
  * Due to binary compatibility, the actual resource numbers