UVM(9)

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:

-   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 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.

INITIALIZATION
void
uvm_init(void);

void
uvm_init_limits(struct lwp *l);

void
uvm_setpagesize(void);

void
uvm_swap_init(void);

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 virtual memory state, machine-dependent physical map, kernel memory allocator, pager and anonymous memory sub-systems, and then enables paging of kernel objects.

uvm_init_limits() initializes process limits for the named process. This is for use by the system startup for process zero, before any other processes are created.

uvm_setpagesize() initializes 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() call (see pmap(9)).

uvm_swap_init() initializes the swap sub-system.

VIRTUAL ADDRESS SPACE MANAGEMENT
int
uvm_map(struct vm_map *map, vaddr_t *startp, vsize_t size, struct uvm_object *uobj, voff_t uoffset, vsize_t align, uvm_flag_t flags);

void
uvm_unmap(struct vm_map *map, vaddr_t start, vaddr_t end);

int
uvm_map_pageable(struct vm_map *map, vaddr_t start, vaddr_t end, bool new_pageable, int lockflags);

bool
uvm_map_checkprot(struct vm_map *map, vaddr_t start, vaddr_t end, vm_prot_t protection);

int
uvm_map_protect(struct vm_map *map, vaddr_t start, vaddr_t end, vm_prot_t new_prot, bool set_max);

int
uvm_deallocate(struct vm_map *map, vaddr_t start, vsize_t size);

struct vmspace *
uvmspace_alloc(vaddr_t min, vaddr_t max, int pageable);

void
uvmspace_exec(struct lwp *l, 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);

void
uvmspace_unshare(struct lwp *l);

bool
uvm_uarea_alloc(vaddr_t *uaddrp);

void
uvm_uarea_free(vaddr_t uaddr);

uvm_map() establishes a valid mapping in map map, which must be unlocked. The new mapping has size size, which must be a multiple of PAGE_SIZE. 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.

align specifies alignment of mapping unless UVM_FLAG_FIXED is specified in flags. align must be a power of 2.

flags passed to uvm_map() are typically created using the UVM_MAPFLAG(vm_prot_t prot, vm_prot_t maxprot, vm_inherit_t inh, int advice, int flags) macro, which uses the following values. The prot and maxprot can take are:

    #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 /* for bss: pad amap to reduce malloc() */
    #define UVM_FLAG_TRYLOCK 0x200000 /* fail if we can not lock map */

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 UVM return value.

uvm_unmap() removes a valid mapping, from start to end, in map map, which must be unlocked.

uvm_map_pageable() changes the pageability of the pages in the range from start to end in map map to new_pageable. uvm_map_pageable() returns a standard UVM return value.

uvm_map_checkprot() checks the protection of the range from start to end in map map against protection. This returns either true or false.

uvm_map_protect() 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 true. This function returns a standard UVM return value.

uvm_deallocate() deallocates kernel memory in map map from address start to start + size.

uvmspace_alloc() allocates and returns a new address space, with ranges from min to max, setting the pageability of the address space to pageable.

uvmspace_exec() either reuses the address space of lwp l 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.

uvmspace_fork() creates and returns a new address space based upon the vm1 address space, typically used when allocating an address space for a child process.

uvmspace_free() lowers the reference count on the address space vm, freeing the data structures if there are no other references.

uvmspace_share() causes process p2 to share the address space of p1.

uvmspace_unshare() ensures that lwp l has its own, unshared address space, by creating a new one if necessary by calling uvmspace_fork().

uvm_uarea_alloc() allocates virtual space for a u-area (i.e., a kernel stack) and stores its virtual address in *uaddrp. The return value is true if the u-area is already backed by wired physical memory, otherwise false.

uvm_uarea_free() frees a u-area allocated with uvm_uarea_alloc(), freeing both the virtual space and any physical pages which may have been allocated to back that virtual space later.

PAGE FAULT HANDLING
int
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
void
uvm_vnp_setsize(struct vnode *vp, voff_t newsize);

void *
ubc_alloc(struct uvm_object *uobj, voff_t offset, vsize_t *lenp, int advice, int flags);

void
ubc_release(void *va, int flags);

int
ubc_uiomove(struct uvm_object *uobj, struct uio *uio, vsize_t todo, int advice, int flags);

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 discarded.

ubc_alloc() creates a kernel mapping of uobj starting at offset offset. The desired length of the mapping is pointed to by lenp, but the actual mapping may be smaller than this. lenp is updated to contain the actual length mapped. advice is the access pattern hint, which must be one of

UVM_ADV_NORMAL
    No hint
UVM_ADV_RANDOM
    Random access hint
UVM_ADV_SEQUENTIAL
    Sequential access hint (from lower offset to higher offset)

The possible flags are

UBC_READ
    Mapping will be accessed for read.
UBC_WRITE
    Mapping will be accessed for write.
UBC_FAULTBUSY
    Fault in window's pages already during mapping operation. Makes sense only for write.

Once the mapping is created, it must be accessed only by methods that can handle faults, such as uiomove() or kcopy(). Page faults on the mapping will result in the object's pager method being called to resolve the fault.

ubc_release() frees the mapping at va for reuse. The mapping may be cached to speed future accesses to the same region of the object. The flags can be any of

UBC_UNMAP
    Do not cache mapping.

ubc_uiomove() allocates an UBC memory window, performs I/O on it and unmaps the window. The advice parameter takes the same values as the respective parameter in ubc_alloc() and the flags parameter takes the same arguments as ubc_alloc() and ubc_unmap(). Additionally, the flag UBC_PARTIALOK can be provided to indicate that it is acceptable to return if an error occurs mid-transfer.

VIRTUAL MEMORY I/O
int
uvm_io(struct vm_map *map, struct uio *uio);

uvm_io() performs the I/O described in uio on the memory described in map.

ALLOCATION OF KERNEL MEMORY
vaddr_t
uvm_km_alloc(struct vm_map *map, vsize_t size, vsize_t align, uvm_flag_t flags);

void
uvm_km_free(struct vm_map *map, vaddr_t addr, vsize_t size, uvm_flag_t flags);

struct vm_map *
uvm_km_suballoc(struct vm_map *map, vaddr_t *min, vaddr_t *max, vsize_t size, bool pageable, bool fixed, struct vm_map *submap);

uvm_km_alloc() allocates size bytes of kernel memory in map map. The first address of the allocated memory range will be aligned according to the align argument (specify 0 if no alignment is necessary). The alignment must be a multiple of page size. The flags is a bitwise inclusive OR of the allocation type and operation flags.

The allocation type should be one of:

UVM_KMF_WIRED
    Wired memory.

UVM_KMF_PAGEABLE
    Demand-paged zero-filled memory.

UVM_KMF_VAONLY
    Virtual address only. No physical pages are mapped in the allocated region. If necessary, it's the caller's responsibility to enter page mappings. It's also the caller's responsibility to clean up the mappings before freeing the address range.

The following operation flags are available:

UVM_KMF_CANFAIL
    Can fail even if UVM_KMF_NOWAIT is not specified and UVM_KMF_WAITVA is specified.

UVM_KMF_ZERO
    Request zero-filled memory. Only supported for UVM_KMF_WIRED. Shouldn't be used with other types.

UVM_KMF_TRYLOCK
    Fail if we can't lock the map.

UVM_KMF_NOWAIT
    Fail immediately if no memory is available.

UVM_KMF_WAITVA
    Sleep to wait for the virtual address resources if needed.

(If neither UVM_KMF_NOWAIT nor UVM_KMF_CANFAIL are specified and UVM_KMF_WAITVA is specified, uvm_km_alloc() will never fail, but rather sleep indefinitely until the allocation succeeds.)

Pageability of the pages allocated with UVM_KMF_PAGEABLE can be changed by uvm_map_pageable(). In that case, the entire range must be changed atomically. Changing a part of the range is not supported.

uvm_km_free() frees the memory range allocated by uvm_km_alloc(). addr must be an address returned by uvm_km_alloc(). map and size must be the same as the ones used for the corresponding uvm_km_alloc(). flags must be the allocation type used for the corresponding uvm_km_alloc().

uvm_km_free() is the only way to free memory ranges allocated by uvm_km_alloc(). uvm_unmap() must not be used.

uvm_km_suballoc() allocates submap 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 true, which causes the min argument to specify the beginning of the address in the submap. If fixed is false, any address of size size will be allocated from map and the start and end addresses returned in min and max. If pageable is true, entries in the map may be paged out.

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 waitok);

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);

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 offset newoff.

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 requests efficiently.

uvm_pglistalloc() 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. 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_pglistalloc().

uvm_pglistfree() frees the list of pages pointed to by list. 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 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.

PROCESSES
void
uvm_pageout(void);

void
uvm_scheduler(void);

void
uvm_swapin(struct lwp *l);

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_swapin() swaps in the named lwp.

PAGE LOAN
int
uvm_loan(struct vm_map *map, vaddr_t start, vsize_t len, void *v, int flags);

void
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().

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);

bool
uvm_chgkprot(void *addr, size_t len, int rw);

void
uvm_kernacc(void *addr, size_t len, int rw);

int
uvm_vslock(struct vmspace *vs, void *addr, size_t len, vm_prot_t prot);

void
uvm_vsunlock(struct vmspace *vs, void *addr, size_t len);

void
uvm_meter(void);

void
uvm_fork(struct lwp *l1, struct lwp *l2, bool shared);

int
uvm_grow(struct proc *p, vaddr_t sp);

void
uvn_findpages(struct uvm_object *uobj, voff_t offset, int *npagesp, struct vm_page **pps, int flags);

void
uvm_swap_stats(int cmd, struct swapent *sep, int sec, register_t *retval);

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 and wakes up the swapper if necessary.

uvm_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

    #define UFP_ALL         0x00    /* return all pages requested */
    #define UFP_NOWAIT      0x01    /* don't sleep */
    #define UFP_NOALLOC     0x02    /* don't allocate new pages */
    #define UFP_NOCACHE     0x04    /* don't return pages which already exist */
    #define UFP_NORDONLY    0x08    /* don't return PG_READONLY pages */

UFP_ALL is a pseudo-flag meaning all requested pages should be returned. UFP_NOWAIT means that we must not sleep. UFP_NOALLOC causes any pages which do not already exist to be skipped. UFP_NOCACHE causes any pages which do already exist to be skipped. UFP_NORDONLY causes any pages which are marked PG_READONLY to be skipped.

uvm_swap_stats() implements the SWAP_STATS and SWAP_OSTATS operation of the swapctl(2) system call. cmd is the requested command, SWAP_STATS or SWAP_OSTATS. The function will copy no more than sec entries in the array pointed by sep. On return, retval holds the actual number of entries copied in the array.

SYSCTL
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 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 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 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 */
    /* 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 pageout */
    int pddeact;    /* number of pages daemon deactivates */

NOTES
uvm_chgkprot() is only available if the kernel has been compiled with options KGDB.

All structure and types whose names begin with “vm_” will be renamed to “uvm_”.

SEE ALSO
swapctl(2), getloadavg(3), kvm(3), sysctl(3), ddb(4), options(4), memoryallocators(9), 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 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.

AUTHORS
Charles D. Cranor <chuck@ccrc.wustl.edu> designed and implemented UVM.

Matthew Green <mrg@eterna.com.au> wrote the swap-space management code and handled the logistical issues involved with merging UVM into the NetBSD source tree.

Chuck Silvers <chuq@chuq.com> implemented the aobj pager, thus allowing UVM to support System V shared memory and process swapping. He also designed and implemented the UBC part of UVM, which uses UVM pages to cache vnode data rather than the traditional buffer cache buffers.