K 10
svn:author
V 7
delphij
K 8
svn:date
V 27
2004-11-24T18:56:13.000000Z
K 7
svn:log
V 1040
Try to close a potential, but serious race in our VM subsystem.

Historically, our contigmalloc1() and contigmalloc2() assumes
that a page in PQ_CACHE can be unconditionally reused by busying
and freeing it.  Unfortunatelly, when object happens to be not
NULL, the code will set m->object to NULL and disregard the fact
that the page is actually in the VM page bucket, resulting in
page bucket hash table corruption and finally, a filesystem
corruption, or a 'page not in hash' panic.

This commit has borrowed the idea taken from DragonFlyBSD's fix
to the VM fix by Matthew Dillon[1].  This version of patch will
do the following checks:

	- When scanning pages in PQ_CACHE, check hold_count and
	  skip over pages that are held temporarily.
	- For pages in PQ_CACHE and selected as candidate of being
	  freed, check if it is busy at that time.

Note:  It seems that this is might be unrelated to kern/72539.

Obtained from:	DragonFlyBSD, sys/vm/vm_contig.c,v 1.11 and 1.12 [1]
Reminded by:	Matt Dillon
Reworked by:	alc
MFC After:	1 week

END