Long ago, revision 1.22 of vm/vm_pager.h introduced a bug. Specifically,

it introduced a check after the call to file system's get pages method
that assumes that the get pages method does not change the array of pages
that is passed to it.  In the case of vnode_pager_generic_getpages(),
this assumption has been incorrect.  The contents of the array of pages
may be shifted by vnode_pager_generic_getpages().  Likely, the problem
has been hidden by vnode_pager_haspage() limiting the set of pages that
are passed to vnode_pager_generic_getpages() such that a shift never
occurs.

The fix implemented herein is to adjust the pointer to the array of pages
rather than shifting the pages within the array.

MFC after: 3 weeks
Fix suggested by: tegge

1 files changed, 1 insertions, 3 deletions

diff --git a/sys/vm/vnode_pager.c b/sys/vm/vnode_pager.c
index e6b88cd..cb4cc39 100644
--- a/sys/vm/vnode_pager.c
+++ b/sys/vm/vnode_pager.c
@@ -857,9 +857,7 @@ vnode_pager_generic_getpages(vp, m, bytecount, reqpage)
 	 * to be zero based...
 	 */
 	if (first != 0) {
-		for (i = first; i < count; i++) {
-			m[i - first] = m[i];
-		}
+		m += first;
 		count -= first;
 		reqpage -= first;
 	}