NAME
uvm
—
virtual memory system external
interface
SYNOPSIS
#include
<sys/param.h>
#include <uvm/uvm.h>
DESCRIPTION
The UVM virtual memory system manages access to the computer's memory resources. User processes and the kernel access these resources through UVM's external interface. UVM's external interface includes functions that:
- initialise UVM subsystems
- manage virtual address spaces
- resolve page faults
- memory map files and devices
- perform uio-based I/O to virtual memory
- allocate and free kernel virtual memory
- allocate and free physical memory
In addition to exporting these services, UVM has two kernel-level processes: pagedaemon and swapper. The pagedaemon process sleeps until physical memory becomes scarce. When that happens, pagedaemon is awoken. It scans physical memory, paging out and freeing memory that has not been recently used. The swapper process swaps in runnable processes that are currently swapped out, if there is room.
There are also several miscellaneous functions.
INITIALISATION
void
uvm_init
(void);
void
uvm_init_limits
(struct
proc *p);
void
uvm_setpagesize
(void);
void
uvm_swap_init
(void);
The
uvm_init
()
function sets up the UVM system at system boot time, after the copyright has
been printed. It initialises global state, the page, map, kernel virtual
memory state, machine-dependent physical map, kernel memory allocator, pager
and anonymous memory subsystems, and then enables paging of kernel objects.
uvm_init
() must be called after machine-dependent
code has registered some free RAM with the
uvm_page_physload
() function.
The
uvm_init_limits
()
function initialises process limits for the named process. This is for use
by the system startup for process zero, before any other processes are
created.
The
uvm_setpagesize
()
function initialises the uvmexp members pagesize (if not already done by
machine-dependent code), pageshift and pagemask. It should be called by
machine-dependent code early in the
pmap_init(9) call.
The
uvm_swap_init
()
function initialises the swap subsystem.
VIRTUAL ADDRESS SPACE MANAGEMENT
int
uvm_map
(vm_map_t
map, vaddr_t
*startp, vsize_t
size, struct uvm_object
*uobj, voff_t
uoffset, vsize_t
alignment, uvm_flag_t
flags);
int
uvm_map_pageable
(vm_map_t
map, vaddr_t start,
vaddr_t end,
boolean_t new_pageable,
int lockflags);
int
uvm_map_pageable_all
(vm_map_t
map, int flags,
vsize_t limit);
boolean_t
uvm_map_checkprot
(vm_map_t
map, vaddr_t start,
vaddr_t end,
vm_prot_t
protection);
int
uvm_map_protect
(vm_map_t
map, vaddr_t start,
vaddr_t end,
vm_prot_t new_prot,
boolean_t set_max);
int
uvm_deallocate
(vm_map_t
map, vaddr_t start,
vsize_t size);
struct vmspace *
uvmspace_alloc
(vaddr_t
min, vaddr_t max,
boolean_t pageable,
boolean_t
remove_holes);
void
uvmspace_exec
(struct
proc *p, vaddr_t
start, vaddr_t
end);
struct vmspace *
uvmspace_fork
(struct
vmspace *vm);
void
uvmspace_free
(struct
vmspace *vm1);
void
uvmspace_share
(struct
proc *p1, struct proc
*p2);
int
UVM_MAPFLAG
(vm_prot_t
prot, vm_prot_t
maxprot, vm_inherit_t
inh, int advice,
int flags);
The
uvm_map
()
function establishes a valid mapping in map map, which
must be unlocked. The new mapping has size size, which
must be in PAGE_SIZE
units. If
alignment is non-zero, it describes the required
alignment of the list, in power-of-two notation. The
uobj and uoffset arguments can
have four meanings. When uobj is
NULL
and uoffset is
UVM_UNKNOWN_OFFSET
,
uvm_map
() does not use the machine-dependent
PMAP_PREFER
function. If
uoffset is any other value, it is used as the hint to
PMAP_PREFER
. When uobj is not
NULL
and uoffset is
UVM_UNKNOWN_OFFSET
,
uvm_map
() finds the offset based upon the virtual
address, passed as startp. If
uoffset is any other value, we are doing a normal
mapping at this offset. The start address of the map will be returned in
startp.
flags passed to
uvm_map
()
are typically created using the
UVM_MAPFLAG
()
macro, which uses the following values. The prot and
maxprot can take the following values:
#define UVM_PROT_MASK 0x07 /* protection mask */ #define UVM_PROT_NONE 0x00 /* protection none */ #define UVM_PROT_ALL 0x07 /* everything */ #define UVM_PROT_READ 0x01 /* read */ #define UVM_PROT_WRITE 0x02 /* write */ #define UVM_PROT_EXEC 0x04 /* exec */ #define UVM_PROT_R 0x01 /* read */ #define UVM_PROT_W 0x02 /* write */ #define UVM_PROT_RW 0x03 /* read-write */ #define UVM_PROT_X 0x04 /* exec */ #define UVM_PROT_RX 0x05 /* read-exec */ #define UVM_PROT_WX 0x06 /* write-exec */ #define UVM_PROT_RWX 0x07 /* read-write-exec */
The values that inh can take are:
#define UVM_INH_MASK 0x30 /* inherit mask */ #define UVM_INH_SHARE 0x00 /* "share" */ #define UVM_INH_COPY 0x10 /* "copy" */ #define UVM_INH_NONE 0x20 /* "none" */ #define UVM_INH_DONATE 0x30 /* "donate" << not used */
The values that advice can take are:
#define UVM_ADV_NORMAL 0x0 /* 'normal' */ #define UVM_ADV_RANDOM 0x1 /* 'random' */ #define UVM_ADV_SEQUENTIAL 0x2 /* 'sequential' */ #define UVM_ADV_MASK 0x7 /* mask */
The values that flags can take are:
#define UVM_FLAG_FIXED 0x010000 /* find space */ #define UVM_FLAG_OVERLAY 0x020000 /* establish overlay */ #define UVM_FLAG_NOMERGE 0x040000 /* don't merge map entries */ #define UVM_FLAG_COPYONW 0x080000 /* set copy_on_write flag */ #define UVM_FLAG_AMAPPAD 0x100000 /* bss: pad amap to reduce malloc() */ #define UVM_FLAG_TRYLOCK 0x200000 /* fail if we can not lock map */ #define UVM_FLAG_HOLE 0x400000 /* no backend */
The UVM_MAPFLAG
macro
arguments can be combined with an or operator. There are several special
purpose macros for checking protection combinations, e.g., the
UVM_PROT_WX
macro. There are also some additional
macros to extract bits from the flags. The
UVM_PROTECTION
, UVM_INHERIT
,
UVM_MAXPROTECTION
and
UVM_ADVICE
macros return the protection,
inheritance, maximum protection and advice, respectively.
uvm_map
()
returns a standard errno.
The
uvm_map_pageable
()
function changes the pageability of the pages in the range from
start to end in map
map to new_pageable. The
uvm_map_pageable_all
()
function changes the pageability of all mapped regions. If
limit is non-zero and
pmap_wired_count
()
is implemented, ENOMEM
is returned if the amount of
wired pages exceed limit. The map is locked on entry
if lockflags contain
UVM_LK_ENTER
, and locked on exit if
lockflags contain UVM_LK_EXIT
.
uvm_map_pageable
() and
uvm_map_pageable_all
() return a standard errno.
The
uvm_map_checkprot
()
function checks the protection of the range from start
to end in map map against
protection. This returns either
TRUE
or FALSE
.
The
uvm_map_protect
()
function changes the protection start to
end in map map to
new_prot, also setting the maximum protection to the
region to new_prot if set_max is
non-zero. This function returns a standard errno.
The
uvm_deallocate
()
function deallocates kernel memory in map map from
address start to start +
size.
The
uvmspace_alloc
()
function allocates and returns a new address space, with ranges from
min to max, setting the
pageability of the address space to pageable. If
remove_holes is non-zero, hardware
‘holes’ in the virtual address space will be removed from the
newly allocated address space.
The
uvmspace_exec
()
function either reuses the address space of process p
if there are no other references to it, or creates a new one with
uvmspace_alloc
(). The range of valid addresses in
the address space is reset to start through
end.
The
uvmspace_fork
()
function creates and returns a new address space based upon the
vm1 address space, typically used when allocating an
address space for a child process.
The
uvmspace_free
()
function lowers the reference count on the address space
vm, freeing the data structures if there are no other
references.
PAGE FAULT HANDLING
int
uvm_fault
(vm_map_t
orig_map, vaddr_t
vaddr, vm_fault_t
fault_type, vm_prot_t
access_type);
The
uvm_fault
()
function is the main entry point for faults. It takes
orig_map as the map the fault originated in, a
vaddr offset into the map the fault occurred,
fault_type describing the type of fault, and
access_type describing the type of access requested.
uvm_fault
() returns a standard errno.
MEMORY MAPPING FILES AND DEVICES
struct uvm_object *
uvn_attach
(void
*arg, vm_prot_t
accessprot);
void
uvm_vnp_setsize
(struct
vnode *vp, voff_t
newsize);
void
uvm_vnp_sync
(struct
mount *mp);
void
uvm_vnp_terminate
(struct
vnode *vp);
boolean_t
uvm_vnp_uncache
(struct
vnode *vp);
The
uvn_attach
()
function attaches a UVM object to vnode arg, creating
the object if necessary. The object is returned.
The
uvm_vnp_setsize
()
function sets the size of vnode vp to
newsize. Caller must hold a reference to the vnode. If
the vnode shrinks, pages no longer used are discarded. This function will be
removed when the file system and VM buffer caches are merged.
The
uvm_vnp_sync
()
function flushes dirty vnodes from either the mount point passed in
mp, or all dirty vnodes if mp is
NULL
. This function will be removed when the file
system and VM buffer caches are merged.
The
uvm_vnp_terminate
()
function frees all VM resources allocated to vnode vp.
If the vnode still has references, it will not be destroyed; however all
future operations using this vnode will fail. This function will be removed
when the file system and VM buffer caches are merged.
The
uvm_vnp_uncache
()
function disables vnode vp from persisting when all
references are freed. This function will be removed when the file system and
UVM caches are unified. Returns true if there is no active vnode.
VIRTUAL MEMORY I/O
int
uvm_io
(vm_map_t
map, struct uio
*uio);
The
uvm_io
()
function performs the I/O described in uio on the
memory described in map.
ALLOCATION OF KERNEL MEMORY
vaddr_t
uvm_km_alloc
(vm_map_t
map, vsize_t
size);
vaddr_t
uvm_km_zalloc
(vm_map_t
map, vsize_t
size);
vaddr_t
uvm_km_alloc1
(vm_map_t
map, vsize_t size,
vsize_t align,
boolean_t zeroit);
vaddr_t
uvm_km_kmemalloc
(vm_map_t
map, struct uvm_object
*obj, vsize_t size,
int flags);
vaddr_t
uvm_km_valloc
(vm_map_t
map, vsize_t
size);
vaddr_t
uvm_km_valloc_wait
(vm_map_t
map, vsize_t
size);
struct vm_map *
uvm_km_suballoc
(vm_map_t
map, vaddr_t *min,
vaddr_t *max ,
vsize_t size,
int flags,
boolean_t fixed,
vm_map_t submap);
void
uvm_km_free
(vm_map_t
map, vaddr_t addr,
vsize_t size);
void
uvm_km_free_wakeup
(vm_map_t
map, vaddr_t addr,
vsize_t size);
The
uvm_km_alloc
()
and
uvm_km_zalloc
()
functions allocate size bytes of wired kernel memory
in map map. In addition to allocation,
uvm_km_zalloc
() zeros the memory. Both of these
functions are defined as macros in terms of
uvm_km_alloc1
(), and should almost always be used in
preference to uvm_km_alloc1
().
The
uvm_km_alloc1
()
function allocates and returns size bytes of wired
memory in the kernel map aligned to the align
boundary, zeroing the memory if the zeroit argument is
non-zero.
The
uvm_km_kmemalloc
()
function allocates and returns size bytes of wired
kernel memory into obj. The flags can be any of:
#define UVM_KMF_NOWAIT 0x1 /* matches M_NOWAIT */ #define UVM_KMF_VALLOC 0x2 /* allocate VA only */ #define UVM_KMF_TRYLOCK UVM_FLAG_TRYLOCK /* try locking only */
The UVM_KMF_NOWAIT
flag causes
uvm_km_kmemalloc
()
to return immediately if no memory is available.
UVM_KMF_VALLOC
causes no pages to be allocated, only
a virtual address. UVM_KMF_TRYLOCK
causes
uvm_km_kmemalloc
() to use
simple_lock_try
()
when locking maps.
The
uvm_km_valloc
()
and
uvm_km_valloc_wait
()
functions return a newly allocated zero-filled address in the kernel map of
size size.
uvm_km_valloc_wait
() will also wait for kernel
memory to become available, if there is a memory shortage.
The
uvm_km_suballoc
()
function allocates submap (with the specified flags,
as described above) from map, creating a new map if
submap is NULL
. The addresses
of the submap can be specified exactly by setting the
fixed argument to non-zero, which causes the
min argument to specify the beginning of the address
in the submap. If fixed is zero, any address of size
size will be allocated from map
and the start and end addresses returned in min and
max.
The
uvm_km_free
()
and
uvm_km_free_wakeup
()
functions free size bytes of memory in the kernel map,
starting at address addr.
uvm_km_free_wakeup
() calls
thread_wakeup
()
on the map before unlocking the map.
ALLOCATION OF PHYSICAL MEMORY
struct vm_page *
uvm_pagealloc
(struct
uvm_object *uobj, voff_t
off, struct vm_anon
*anon, int
flags);
void
uvm_pagerealloc
(struct
vm_page *pg, struct
uvm_object *newobj,
voff_t newoff);
void
uvm_pagefree
(struct
vm_page *pg);
int
uvm_pglistalloc
(psize_t
size, paddr_t low,
paddr_t high,
paddr_t alignment,
paddr_t boundary,
struct pglist *rlist,
int nsegs,
int flags);
void
uvm_pglistfree
(struct
pglist *list);
void
uvm_page_physload
(vaddr_t
start, vaddr_t end,
vaddr_t avail_start,
vaddr_t avail_end,
int free_list);
The
uvm_pagealloc
()
function allocates a page of memory at virtual address
off in either the object uobj or
the anonymous memory anon, which must be locked by the
caller. Only one of anon and
uobj can be non NULL
. The
flags can be any of:
#define UVM_PGA_USERESERVE 0x0001 /* ok to use reserve pages */ #define UVM_PGA_ZERO 0x0002 /* returned page must be zeroed */
The UVM_PGA_USERESERVE
flag means to
allocate a page even if that will result in the number of free pages being
lower than uvmexp.reserve_pagedaemon
(if the current
thread is the pagedaemon) or uvmexp.reserve_kernel
(if the current thread is not the pagedaemon). The
UVM_PGA_ZERO
flag causes the returned page to be
filled with zeroes, either by allocating it from a pool of pre-zeroed pages
or by zeroing it in-line as necessary.
The
uvm_pagerealloc
()
function reallocates page pg to a new object
newobj, at a new offset newoff,
and returns NULL
when no page can be found.
The
uvm_pagefree
()
function frees the physical page pg.
The
uvm_pglistalloc
()
function allocates a list of pages for size size byte
under various constraints. low and
high describe the lowest and highest addresses
acceptable for the list. If alignment is non-zero, it
describes the required alignment of the list, in power-of-two notation. If
boundary is non-zero, no segment of the list may cross
this power-of-two boundary, relative to zero. nsegs is
the maximum number of physically contiguous segments. The allocated memory
is returned in the rlist list. The
flags can be any of:
#define UVM_PLA_WAITOK 0x0001 /* may sleep */ #define UVM_PLA_NOWAIT 0x0002 /* can't sleep */ #define UVM_PLA_ZERO 0x0004 /* zero all pages before returning */
The UVM_PLA_WAITOK
flag means that the
function may sleep while trying to allocate the list of pages (this is
currently ignored). Conversely, the UVM_PLA_NOWAIT
flag signifies that the function may not sleep while allocating. It is an
error not to provide one of the above flags. Optionally, one may also
specify the UVM_PLA_ZERO
flag to receive zeroed
memory in the page list.
The
uvm_pglistfree
()
function frees the list of pages pointed to by
list.
The
uvm_page_physload
()
function loads physical memory segments into VM space on the specified
free_list. uvm_page_physload
()
must be called at system boot time to set up physical memory management
pages. The arguments describe the start and
end of the physical addresses of the segment, and the
available start and end addresses of pages not already in use.
PROCESSES
void
uvm_pageout
(void
*arg);
void
uvm_scheduler
(void);
void
uvm_swapin
(struct
proc *p);
The
uvm_pageout
()
function is the main loop for the page daemon. The arg
argument is ignored.
The
uvm_scheduler
()
function is the process zero main loop, which is to be called after the
system has finished starting other processes.
uvm_scheduler
() handles the swapping in of runnable,
swapped out processes in priority order.
The
uvm_swapin
()
function swaps in the named process.
MISCELLANEOUS FUNCTIONS
struct uvm_object *
uao_create
(vsize_t
size, int
flags);
void
uao_detach
(struct
uvm_object *uobj);
void
uao_reference
(struct
uvm_object *uobj);
boolean_t
uvm_chgkprot
(caddr_t
addr, size_t len,
int rw);
void
uvm_kernacc
(caddr_t
addr, size_t len,
int rw);
void
uvm_vslock
(struct
proc *p, caddr_t
addr, size_t len,
vm_prot_t
access_type);
void
uvm_vsunlock
(struct
proc *p, caddr_t
addr, size_t
len);
void
uvm_meter
();
int
uvm_sysctl
(int
*name, u_int
namelen, void
*oldp, size_t
*oldlenp, void *newp
, size_t newlen,
struct proc *p);
void
uvm_fork
(struct
proc *p1, struct proc
*p2, boolean_t
shared, void
*stack, size_t
stacksize, void
(*func)(void *arg), void
*arg);
int
uvm_grow
(struct
proc *p, vaddr_t
sp);
int
uvm_coredump
(struct
proc *p, struct vnode
*vp, struct ucred
*cred, struct core
*chdr);
The
uao_create
(),
uao_detach
()
and uao_reference
() functions operate on anonymous
memory objects, such as those used to support System V shared memory.
uao_create
() returns an object of size
size with flags:
#define UAO_FLAG_KERNOBJ 0x1 /* create kernel object */ #define UAO_FLAG_KERNSWAP 0x2 /* enable kernel swap */
uao_reference
()
creates an additional reference to the named anonymous memory object.
uao_detach
()
removes a reference from the named anonymous memory object, destroying it if
removing the last reference.
The
uvm_chgkprot
()
function changes the protection of kernel memory from
addr to addr + len to the value
of rw. This is primarily useful for debuggers, for
setting breakpoints. This function is only available with options
KGDB
.
The
uvm_kernacc
()
function checks the access at address addr to
addr + len for rw access, in the
kernel address space.
The
uvm_vslock
()
and
uvm_vsunlock
()
functions control the wiring and unwiring of pages for process
p from addr to
addr + len. The access_type
argument of uvm_vslock
() is passed to
uvm_fault
(). These functions are normally used to
wire memory for I/O.
The
uvm_meter
()
function calculates the load average and wakes up the swapper if
necessary.
The
uvm_sysctl
()
function provides support for the CTL_VM
domain of
the sysctl(3) hierarchy. uvm_sysctl
() handles the
VM_LOADAVG
, VM_METER
and
VM_UVMEXP
calls, which return the current load
averages, calculates current VM totals, and returns the uvmexp structure
respectively. The load averages are accessed from userland using the
getloadavg(3) function. The uvmexp structure has all global
state of the UVM system, and has the following members:
/* vm_page constants */ int pagesize; /* size of a page (PAGE_SIZE): must be power of 2 */ int pagemask; /* page mask */ int pageshift; /* page shift */ /* vm_page counters */ int npages; /* number of pages we manage */ int free; /* number of free pages */ int active; /* number of active pages */ int inactive; /* number of pages that we free'd but may want back */ int paging; /* number of pages in the process of being paged out */ int wired; /* number of wired pages */ int zeropages; /* number of zero'd pages */ int reserve_pagedaemon; /* number of pages reserved for pagedaemon */ int reserve_kernel; /* number of pages reserved for kernel */ int anonpages; /* number of pages used by anon pagers */ int vnodepages; /* number of pages used by vnode page cache */ int vtextpages; /* number of pages used by vtext vnodes */ /* pageout params */ int freemin; /* min number of free pages */ int freetarg; /* target number of free pages */ int inactarg; /* target number of inactive pages */ int wiredmax; /* max number of wired pages */ int anonmin; /* min threshold for anon pages */ int vtextmin; /* min threshold for vtext pages */ int vnodemin; /* min threshold for vnode pages */ int anonminpct; /* min percent anon pages */ int vtextminpct;/* min percent vtext pages */ int vnodeminpct;/* min percent vnode pages */ /* swap */ int nswapdev; /* number of configured swap devices in system */ int swpages; /* number of PAGE_SIZE'ed swap pages */ int swpginuse; /* number of swap pages in use */ int swpgonly; /* number of swap pages in use, not also in RAM */ int nswget; /* number of times fault calls uvm_swap_get() */ int nanon; /* number total of anon's in system */ int nanonneeded;/* number of anons currently needed */ int nfreeanon; /* number of free anon's */ /* stat counters */ int faults; /* page fault count */ int traps; /* trap count */ int intrs; /* interrupt count */ int swtch; /* context switch count */ int softs; /* software interrupt count */ int syscalls; /* system calls */ int pageins; /* pagein operation count */ /* pageouts are in pdpageouts below */ int swapins; /* swapins */ int swapouts; /* swapouts */ int pgswapin; /* pages swapped in */ int pgswapout; /* pages swapped out */ int forks; /* forks */ int forks_ppwait; /* forks where parent waits */ int forks_sharevm; /* forks where vmspace is shared */ int pga_zerohit; /* pagealloc where zero wanted and zero was available */ int pga_zeromiss; /* pagealloc where zero wanted and zero not available */ int zeroaborts; /* number of times page zeroing was aborted */ /* fault subcounters */ int fltnoram; /* number of times fault was out of ram */ int fltnoanon; /* number of times fault was out of anons */ int fltpgwait; /* number of times fault had to wait on a page */ int fltpgrele; /* number of times fault found a released page */ int fltrelck; /* number of times fault relock called */ int fltrelckok; /* number of times fault relock is a success */ int fltanget; /* number of times fault gets anon page */ int fltanretry; /* number of times fault retrys an anon get */ int fltamcopy; /* number of times fault clears "needs copy" */ int fltnamap; /* number of times fault maps a neighbor anon page */ int fltnomap; /* number of times fault maps a neighbor obj page */ int fltlget; /* number of times fault does a locked pgo_get */ int fltget; /* number of times fault does an unlocked get */ int flt_anon; /* number of times fault anon (case 1a) */ int flt_acow; /* number of times fault anon cow (case 1b) */ int flt_obj; /* number of times fault is on object page (2a) */ int flt_prcopy; /* number of times fault promotes with copy (2b) */ int flt_przero; /* number of times fault promotes with zerofill (2b) */ /* daemon counters */ int pdwoke; /* number of times daemon woke up */ int pdrevs; /* number of times daemon rev'd clock hand */ int pdswout; /* number of times daemon called for swapout */ int pdfreed; /* number of pages daemon freed since boot */ int pdscans; /* number of pages daemon scanned since boot */ int pdanscan; /* number of anonymous pages scanned by daemon */ int pdobscan; /* number of object pages scanned by daemon */ int pdreact; /* number of pages daemon reactivated since boot */ int pdbusy; /* number of times daemon found a busy page */ int pdpageouts; /* number of times daemon started a pageout */ int pdpending; /* number of times daemon got a pending pagout */ int pddeact; /* number of pages daemon deactivates */ int pdreanon; /* anon pages reactivated due to min threshold */ int pdrevnode; /* vnode pages reactivated due to min threshold */ int pdrevtext; /* vtext pages reactivated due to min threshold */ int fpswtch; /* FPU context switches */ int kmapent; /* number of kernel map entries */
The
uvm_fork
()
function forks a virtual address space for process' (old)
p1 and (new) p2. If the
shared argument is non zero, p1 shares its address
space with p2, otherwise a new address space is created. The
stack, stacksize,
func and arg arguments are
passed to the machine-dependent
cpu_fork
()
function. The uvm_fork
() function currently has no
return value, and thus cannot fail.
The
uvm_grow
()
function increases the stack segment of process p to
include sp.
The
uvm_coredump
()
function generates a coredump on vnode vp for process
p with credentials cred and core
header description in chdr.
NOTES
The structure and types whose names begin with “vm_” were named so UVM could coexist with BSD VM during the early development stages. They will be renamed to “uvm_”.
SEE ALSO
getloadavg(3), kvm(3), sysctl(3), ddb(4), options(4), pmap(9)
HISTORY
UVM is a new VM system developed at Washington University in St. Louis (Missouri). UVM's roots lie partly in the Mach-based 4.4BSD VM system, the FreeBSD VM system, and the SunOS4 VM system. UVM's basic structure is based on the 4.4BSD VM system. UVM's new anonymous memory system is based on the anonymous memory system found in the SunOS4 VM (as described in papers published by Sun Microsystems, Inc.). UVM also includes a number of features new to BSD including page loanout, map entry passing, simplified copy-on-write, and clustered anonymous memory pageout. UVM is also further documented in an August 1998 dissertation by Charles D. Cranor.
UVM appeared in OpenBSD 2.9.
AUTHORS
Charles D. Cranor ⟨chuck@ccrc.wustl.edu⟩ designed and implemented UVM.
Matthew Green ⟨mrg@eterna.com.au⟩ wrote the swap-space management code.
Chuck Silvers ⟨chuq@chuq.com⟩ implemented the aobj pager, thus allowing UVM to support System V shared memory and process swapping.
Artur Grabowski ⟨art@openbsd.org⟩ handled the logistical issues involved with merging UVM into the OpenBSD source tree.
BUGS
The uvm_fork
() function should be able to
fail in low memory conditions.