Make collapse synchronization more explicit and allow it to complete during

paging.

Shadow objects are marked with a COLLAPSING flag while they are collapsing with
their backing object.  This gives us an explicit test rather than overloading
paging-in-progress.  While split is on-going we mark an object with SPLIT.
These two operations will modify the swap tree so they must be serialized
and swap_pager_getpages() can now directly detect these conditions and page
more conservatively.

Callers to vm_object_collapse() now will reliably wait for a collapse to finish
so that the backing chain is as short as possible before other decisions are
made that may inflate the object chain.  For example, split, coalesce, etc.
It is now safe to run fault concurrently with collapse.  It is safe to increase
or decrease paging in progress with no lock so long as there is another valid
ref on increase.

This change makes collapse more reliable as a secondary benefit.  The primary
benefit is making it safe to drop the object lock much earlier in fault or
never acquire it at all.

This was tested with a new shadow chain test script that uncovered long
standing bugs and will be integrated with stress2.

Reviewed by:	kib, markj
Differential Revision:	https://reviews.freebsd.org/D22908

1 files changed, 2 insertions, 0 deletions

diff --git a/sys/vm/vm_object.h b/sys/vm/vm_object.h
index b5b6e0c70dc1..780f7c7e83b7 100644
--- a/sys/vm/vm_object.h
+++ b/sys/vm/vm_object.h
@@ -190,6 +190,8 @@ struct vm_object {
 #define	OBJ_SIZEVNLOCK	0x0040		/* lock vnode to check obj size */
 #define	OBJ_PG_DTOR	0x0080		/* dont reset object, leave that for dtor */
 #define	OBJ_TMPFS_NODE	0x0200		/* object belongs to tmpfs VREG node */
+#define	OBJ_SPLIT	0x0400		/* object is being split */
+#define	OBJ_COLLAPSING	0x0800		/* Parent of collapse. */
 #define	OBJ_COLORED	0x1000		/* pg_color is defined */
 #define	OBJ_ONEMAPPING	0x2000		/* One USE (a single, non-forked) mapping flag */
 #define	OBJ_SHADOWLIST	0x4000		/* Object is on the shadow list. */