This commit represents work mainly submitted by Tor and slightly modified

by myself.  It solves a serious vm_map corruption problem that can occur
with the buffer cache when block sizes > 64K are used.  This code has been
heavily tested in -stable but only tested somewhat on -current.  An MFC
will occur in a few days.  My additions include the vm_map_simplify_entry()
and minor buffer cache boundry case fix.

Make the buffer cache use a system map for buffer cache KVM rather then a
normal map.

Ensure that VM objects are not allocated for system maps.  There were cases
where a buffer map could wind up with a backing VM object -- normally
harmless, but this could also result in the buffer cache blocking in places
where it assumes no blocking will occur, possibly resulting in corrupted
maps.

Fix a minor boundry case in the buffer cache size limit is reached that
could result in non-optimal code.

Add vm_map_simplify_entry() calls to prevent 'creeping proliferation'
of vm_map_entry's in the buffer cache's vm_map.  Previously only a simple
linear optimization was made.  (The buffer vm_map typically has only a
handful of vm_map_entry's.  This stabilizes it at that level permanently).

PR: 20609
Submitted by: (Tor Egge) tegge

1 files changed, 13 insertions, 6 deletions

diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 14ae4a8..a0d693c 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -1235,9 +1235,8 @@ brelse(struct buf * bp)
 		bufcountwakeup();
 
 	/*
-	 * Something we can maybe free.
+	 * Something we can maybe free or reuse
 	 */
-
 	if (bp->b_bufsize || bp->b_kvasize)
 		bufspacewakeup();
 
@@ -1304,7 +1303,7 @@ bqrelse(struct buf * bp)
 	}
 
 	/*
-	 * Something we can maybe wakeup
+	 * Something we can maybe free or reuse.
 	 */
 	if (bp->b_bufsize && !(bp->b_flags & B_DELWRI))
 		bufspacewakeup();
@@ -1551,10 +1550,13 @@ restart:
 		}
 
 		/*
-		 * Nada.  If we are allowed to allocate an EMPTY 
-		 * buffer, go get one.
+		 * If we could not find or were not allowed to reuse a
+		 * CLEAN buffer, check to see if it is ok to use an EMPTY
+		 * buffer.  We can only use an EMPTY buffer if allocating
+		 * its KVA would not otherwise run us out of buffer space.
 		 */
-		if (nbp == NULL && defrag == 0 && bufspace < hibufspace) {
+		if (nbp == NULL && defrag == 0 &&
+		    bufspace + maxsize < hibufspace) {
 			nqindex = QUEUE_EMPTY;
 			nbp = TAILQ_FIRST(&bufqueues[QUEUE_EMPTY]);
 		}
@@ -1686,6 +1688,11 @@ restart:
 			goto restart;
 		}
 
+		/*
+		 * If we are overcomitted then recover the buffer and its
+		 * KVM space.  This occurs in rare situations when multiple
+		 * processes are blocked in getnewbuf() or allocbuf().
+		 */
 		if (bufspace >= hibufspace)
 			flushingbufs = 1;
 		if (flushingbufs && bp->b_kvasize != 0) {