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UVM(9) NetBSD Kernel Developer's Manual UVM(9)
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uvm -- virtual memory system external interface
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
- initialize UVM sub-systems
- 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 pro-
cesses: 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.
uvm_init_limits(struct lwp *l);
uvm_init() sets up the UVM system at system boot time, after the console
has been setup. It initializes global state, the page, map, kernel vir-
tual memory state, machine-dependent physical map, kernel memory alloca-
tor, pager and anonymous memory sub-systems, and then enables paging of
uvm_init_limits() initializes process limits for the named process. This
is for use by the system startup for process zero, before any other pro-
cesses are created.
uvm_md_init() does early boot initialization. This currently includes:
uvm_setpagesize() which initializes the uvmexp members pagesize (if not
already done by machine-dependent code), pageshift and pagemask.
uvm_physseg_init() which initialises the uvm_hotplug(9) subsystem. It
should be called by machine-dependent code early in the pmap_init() call
uvm_swap_init() initializes the swap sub-system.
VIRTUAL ADDRESS SPACE MANAGEMENT
PAGE FAULT HANDLING
uvm_fault(struct vm_map *orig_map, vaddr_t vaddr, vm_prot_t access_type);
uvm_fault() 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, and access_type describing the type of access requested.
uvm_fault() returns a standard UVM return value.
MEMORY MAPPING FILES AND DEVICES
VIRTUAL MEMORY I/O
uvm_io(struct vm_map *map, struct uio *uio);
uvm_io() performs the I/O described in uio on the memory described in
ALLOCATION OF KERNEL MEMORY
ALLOCATION OF PHYSICAL MEMORY
struct vm_page *
uvm_pagealloc(struct uvm_object *uobj, voff_t off, struct vm_anon *anon,
uvm_pagerealloc(struct vm_page *pg, struct uvm_object *newobj, voff_t
uvm_pagefree(struct vm_page *pg);
uvm_pglistalloc(psize_t size, paddr_t low, paddr_t high, paddr_t
alignment, paddr_t boundary, struct pglist *rlist, int nsegs, int
uvm_pglistfree(struct pglist *list);
uvm_page_physload(paddr_t start, paddr_t end, paddr_t avail_start,
paddr_t avail_end, int free_list);
uvm_pagealloc() 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 uobj and anon can be non NULL. Returns NULL
when no page can be found. 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 zero'd */
UVM_PGA_USERESERVE 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). UVM_PGA_ZERO 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.
uvm_pagerealloc() reallocates page pg to a new object newobj, at a new
uvm_pagefree() frees the physical page pg. If the content of the page is
known to be zero-filled, caller should set PG_ZERO in pg->flags so that
the page allocator will use the page to serve future UVM_PGA_ZERO
uvm_pglistalloc() allocates a list of pages for size size byte under var-
ious 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. If waitok is non-zero, the function may sleep until enough
memory is available. (It also may give up in some situations, so a non-
zero waitok does not imply that uvm_pglistalloc() cannot return an
error.) The allocated memory is returned in the rlist list; the caller
has to provide storage only, the list is initialized by
uvm_pglistfree() frees the list of pages pointed to by list. If the con-
tent of the page is known to be zero-filled, caller should set PG_ZERO in
pg->flags so that the page allocator will use the page to serve future
UVM_PGA_ZERO requests efficiently.
uvm_page_physload() loads physical memory segments into VM space on the
specified free_list. It 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. If a system has memory banks
of different speeds the slower memory should be given a higher free_list
uvm_pageout() is the main loop for the page daemon.
uvm_scheduler() is the process zero main loop, which is to be called
after the system has finished starting other processes. It handles the
swapping in of runnable, swapped out processes in priority order.
uvm_loan(struct vm_map *map, vaddr_t start, vsize_t len, void *v, int
uvm_unloan(void *v, int npages, int flags);
uvm_loan() loans pages in a map out to anons or to the kernel. map
should be unlocked, start and len should be multiples of PAGE_SIZE.
Argument flags should be one of
#define UVM_LOAN_TOANON 0x01 /* loan to anons */
#define UVM_LOAN_TOPAGE 0x02 /* loan to kernel */
v should be pointer to array of pointers to struct anon or struct
vm_page, as appropriate. The caller has to allocate memory for the array
and ensure it's big enough to hold len / PAGE_SIZE pointers. Returns 0
for success, or appropriate error number otherwise. Note that wired
pages can't be loaned out and uvm_loan() will fail in that case.
uvm_unloan() kills loans on pages or anons. The v must point to the
array of pointers initialized by previous call to uvm_loan(). npages
should match number of pages allocated for loan, this also matches number
of items in the array. Argument flags should be one of
#define UVM_LOAN_TOANON 0x01 /* loan to anons */
#define UVM_LOAN_TOPAGE 0x02 /* loan to kernel */
and should match what was used for previous call to uvm_loan().
struct uvm_object *
uao_create(vsize_t size, int flags);
uao_detach(struct uvm_object *uobj);
uao_reference(struct uvm_object *uobj);
uvm_chgkprot(void *addr, size_t len, int rw);
uvm_kernacc(void *addr, size_t len, int rw);
uvm_vslock(struct vmspace *vs, void *addr, size_t len, vm_prot_t prot);
uvm_vsunlock(struct vmspace *vs, void *addr, size_t len);
uvm_proc_fork(struct proc *p1, struct proc *p2, bool shared);
uvm_grow(struct proc *p, vaddr_t sp);
uvn_findpages(struct uvm_object *uobj, voff_t offset, int *npagesp,
struct vm_page **pps, int flags);
uvm_vnp_setsize(struct vnode *vp, voff_t newsize);
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 */
which can only be used once each at system boot time. 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.
uvm_chgkprot() 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.
uvm_kernacc() checks the access at address addr to addr + len for rw
access in the kernel address space.
uvm_vslock() and uvm_vsunlock() control the wiring and unwiring of pages
for process p from addr to addr + len. These functions are normally used
to wire memory for I/O.
uvm_meter() calculates the load average.
uvm_proc_fork() 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. This function
currently has no return value, and thus cannot fail. In the future, this
function will be changed to allow it to fail in low memory conditions.
uvm_grow() increases the stack segment of process p to include sp.
uvn_findpages() looks up or creates pages in uobj at offset offset, marks
them busy and returns them in the pps array. Currently uobj must be a
vnode object. The number of pages requested is pointed to by npagesp,
and this value is updated with the actual number of pages returned. The
flags can be any bitwise inclusive-or of:
UFP_ALL Zero pseudo-flag meaning return all pages.
UFP_NOWAIT Don't sleep -- yield NULL for busy pages or for
uncached pages for which allocation would sleep.
UFP_NOALLOC Don't allocate -- yield NULL for uncached pages.
UFP_NOCACHE Don't use cached pages -- yield NULL instead.
UFP_NORDONLY Don't yield read-only pages -- yield NULL for
pages marked PG_READONLY.
UFP_DIRTYONLY Don't yield clean pages -- stop early at the
first clean one. As a side effect, mark yielded
dirty pages clean. Caller must write them to
permanent storage before unbusying.
UFP_BACKWARD Traverse pages in reverse order. If
uvn_findpages() returns early, it will have
filled *npagesp entries at the end of pps rather
than the beginning.
uvm_vnp_setsize() 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
The atop() macro converts a physical address pa into a page number. The
ptoa() macro does the opposite by converting a page number pn into a
round_page() and trunc_page() macros return a page address boundary from
rounding address up and down, respectively, to the nearest page boundary.
These macros work for either addresses or byte counts.
UVM provides support for the CTL_VM domain of the sysctl(3) hierarchy.
It handles the VM_LOADAVG, VM_METER, VM_UVMEXP, and VM_UVMEXP2 nodes,
which return the current load averages, calculates current VM totals,
returns the uvmexp structure, and a kernel version independent view of
the uvmexp structure, respectively. It also exports a number of tunables
that control how much VM space is allowed to be consumed by various
tasks. The load averages are typically 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 paging; /* number of pages in the process of being paged out */
int wired; /* number of wired pages */
int reserve_pagedaemon; /* number of pages reserved for pagedaemon */
int reserve_kernel; /* number of pages reserved for kernel */
/* 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 */
/* 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 nswget; /* number of times fault calls uvm_swap_get() */
int nanon; /* number total of anon's in system */
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 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 */
/* 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 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 pageout */
int pddeact; /* number of pages daemon deactivates */
uvm_chgkprot() is only available if the kernel has been compiled with
All structure and types whose names begin with ``vm_'' will be renamed to
swapctl(2), getloadavg(3), kvm(3), sysctl(3), ddb(4), options(4),
memoryallocators(9), pmap(9), ubc(9), uvm_km(9), uvm_map(9)
Charles D. Cranor and Gurudatta M. Parulkar, "The UVM Virtual Memory
System", Proceedings of the USENIX Annual Technical Conference, USENIX
117-130, June 6-11, 1999.
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 SunOS 4 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 SunOS 4 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 NetBSD 1.4.
Charles D. Cranor <firstname.lastname@example.org> designed and implemented UVM.
Matthew Green <email@example.com> wrote the swap-space management code
and handled the logistical issues involved with merging UVM into the
NetBSD source tree.
Chuck Silvers <firstname.lastname@example.org> implemented the aobj pager, thus allowing
UVM to support System V shared memory and process swapping.
NetBSD 9.0 March 23, 2015 NetBSD 9.0