DragonFlyBSD

A panic was seen on x86-64 where vm_pageout_active_scan() had removed a page from the active queue and found the object pointer nulled. The offending code section is roughly at line 1245 of vm_pageout.c (vm_pageout_active_scan(); we have found & busied the page, unlocked the queues; testing refcount).

(panic seen by sephe@)

1) vm_pageout_scan_active got an active page off of PQ_ACTIVE. The
page was referenced, busy, PG_CLEANCHKed, and PG_NEED_COMMITed. The
page's m->object field was NULL, but m->pindex was not, pointing to a
recent vm_page_remove() {which does not erase pindex}.

2) At first glance, it looks like vm_pageout_scan_active() and
vm_page_remove() might have a race; specifically:

vm_pageout_scan_active
...
* Lock page queue for page we selected
* Try to busy page (PG_BUSY set)
* If successful, unlock page queue for the page and the
page spinlock too
* yield()

vm_page_remove()

* hold the vm_object for the pg
This should be safe, because
the page was PG_BUSY. But
perhaps one caller is not
correctly busying the page?

* spinlock the page
* remove the page from the obj
* spinunlock the page
* drop the object

* find m->object NULL ! Surprise!