Don't remove the virtual-to-physical mapping when an sf_buf is freed.

Instead, allow the mapping to persist, but add the sf_buf to a free list.
If a later sendfile(2) or zero-copy send resends the same physical page,
perhaps with the same or different contents, then the mapping overhead is
avoided and the sf_buf is simply removed from the free list.

In other words, the i386 sf_buf implementation now behaves as a cache of
virtual-to-physical translations using an LRU replacement policy on
inactive sf_bufs.  This is similar in concept to a part of
http://www.cs.princeton.edu/~yruan/debox/ patch, but much simpler in
implementation.  Note: none of this is required on alpha, amd64, or ia64.
They now use their direct virtual-to-physical mapping to avoid any
emphemeral mapping overheads in their sf_buf implementations.

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/i386/include/sf_buf.h b/sys/i386/include/sf_buf.h
index 2784002..7d438f0 100644
--- a/sys/i386/include/sf_buf.h
+++ b/sys/i386/include/sf_buf.h
@@ -35,6 +35,7 @@ struct vm_page;
 
 struct sf_buf {
 	LIST_ENTRY(sf_buf) list_entry;	/* list of buffers */
+	TAILQ_ENTRY(sf_buf) free_entry;	/* list of buffers */
 	struct		vm_page *m;	/* currently mapped page */
 	vm_offset_t	kva;		/* va of mapping */
 	int		ref_count;	/* usage of this mapping */