Subject: can MEXTADD() storage be remapped ?
To: None <>
From: Manuel Bouyer <>
List: tech-kern
Date: 05/23/2005 18:16:45
I'm wondering if the MMU mapping for a mbuf external storage may be
modified later by the kernel (or new mappings be added, deleted, etc ...)

In if_xennet, data received from the interface is passed to the kernel
in a mbuf with external storage (MEXTADD). In the external storage
free  routine (xennet_rx_mbuf_free()), there is a comment saying
that that there may be some MMU operation pending for this buffer
(in Xen, some MMU operations are delayed via a MMU update queue),
and the MMU update queue must be flushed. This cause problems,
and I'm wondering if there can ever be MMU operations queued for
these pages, once they have been MEXTADD()'d to a mbuf and passed to the
network stack. If would make my life much easier if the anserw was "no" :)

Manuel Bouyer <>
     NetBSD: 26 ans d'experience feront toujours la difference