---

 25-akpm/mm/rmap.c |   19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)

diff -puN mm/rmap.c~try_to_unmap_cluster-comment mm/rmap.c
--- 25/mm/rmap.c~try_to_unmap_cluster-comment	Mon May 10 15:29:02 2004
+++ 25-akpm/mm/rmap.c	Mon May 10 15:29:02 2004
@@ -594,8 +594,23 @@ out:
 }
 
 /*
- * try_to_unmap_cluster is only used on VM_NONLINEAR shared object vmas,
- * in which objrmap is unable to predict where a page will be found.
+ * objrmap doesn't work for nonlinear VMAs because the assumption that
+ * offset-into-file correlates with offset-into-virtual-addresses does not hold.
+ * Consequently, given a particular page and its ->index, we cannot locate the
+ * ptes which are mapping that page without an exhaustive linear search.
+ *
+ * So what this code does is a mini "virtual scan" of each nonlinear VMA which
+ * maps the file to which the target page belongs.  The ->vm_private_data field
+ * holds the current cursor into that scan.  Successive searches will circulate
+ * around the vma's virtual address space.
+ *
+ * So as more replacement pressure is applied to the pages in a nonlinear VMA,
+ * more scanning pressure is placed against them as well.   Eventually pages
+ * will become fully unmapped and are eligible for eviction.
+ *
+ * For very sparsely populated VMAs this is a little inefficient - chances are
+ * there there won't be many ptes located within the scan cluster.  In this case
+ * maybe we could scan further - to the end of the pte page, perhaps.
  */
 #define CLUSTER_SIZE	(32 * PAGE_SIZE)
 #if     CLUSTER_SIZE  >	PMD_SIZE

_