From: utzoo!decvax!harpo!eagle!mit-vax!masscomp!tjt
Newsgroups: net.unix-wizards
Title: Re: File System Limit
Article-I.D.: masscomp.116
Posted: Fri Apr  1 22:11:03 1983
Received: Sun Apr  3 02:32:20 1983
References: rayssd.1027

The 4BSD paging algorithm keeps a table of information about each
page in physical memory (except those allocated very early in the
startup procedure).  This structure (in <sys/cmap.h>) includes the
field c_mdev:4 to identify which mounted file system a page came
from, and is used for retaining text pages of pageable text files
(magic number 0413) after the page has been released.  I think it is
also for identifying "reclaimable" pages. i.e. pages made
inaccessable to a process but retained in memory.  This is the
mechanism used to simulate a "page referenced" bit.  As a special
added attraction, if c_mdev is equal to NMOUNT, the page came from
the swap area (i.e. it's a data or stack page).  In any case, if you
make NMOUNT >= 16, you had better add more bits to the c_mdev field
as well.