UVM(9)

NAME
uvm - virtual memory system external interface

SYNOPSIS
#include <sys/param.h>
#include <vm/vm.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 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.

INITIALISATION

void uvm_init() void uvm_init_limits(struct proc *p) void uvm_setpagesize() void uvm_swap_init()

uvm_init() 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 sub-systems, and then enables paging of kernel objects.

uvm_init_limits() initialises 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() 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.

uvm_swap_init() initialises the swap sub-system.

VIRTUAL ADDRESS SPACE MANAGEMENT

int uvm_map(vm_map_t map, vaddr_t *startp, vsize_t size, struct uvm_object *uobj, voff_t uoffset, uvm_flag_t flags) int uvm_map_pageable(vm_map_t map, vaddr_t start, vaddr_t end, boolean_t new_pageable) 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, int pageable) void uvmspace_exec(struct proc *p) 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 proc *p)

uvm_map() establishes a valid mapping in map map, which must be unlocked. The new mapping has size size, which must be in PAGE_SIZE units. 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-dependant 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(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_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 non-zero. 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 process p if there are no other references to it, or creates a new one with uvmspace_alloc().

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 process p has its own, unshared address space, by creating a new one if necessary by calling uvmspace_fork().

PAGE FAULT HANDLING

int uvm_fault(vm_map_t orig_map, vaddr_t vaddr, vm_fault_t fault_type, 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 occured, fault_type describing the type of fault, and access_type describing the type of access requested. uvm_fault() returns a standard UVM return value.

MEMORY MAPPING FILES AND DEVICES

struct uvm_object * uvn_attach(void *arg, vm_prot_t accessprot) void uvm_vnp_setsize(struct vnode *vp, u_quad_t newsize) void uvm_vnp_sync(struct mount *mp) void uvm_vnp_terminate(struct vnode *vp) boolean_t uvm_vnp_uncache(struct vnode *vp)

uvn_attach() attaches a UVM object to vnode arg, creating the object if necessary. The object is returned.

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. This function will be removed when the filesystem and VM buffer caches are merged.

uvm_vnp_sync() 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 filesystem and VM buffer caches are merged.

uvm_vnp_terminate() 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 filesystem and VM buffer caches are merged.

uvm_vnp_uncache() 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)

uvm_io() 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, 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, boolean_t pageable, 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)

uvm_km_alloc() and uvm_km_zalloc() 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().

uvm_km_alloc1() allocates and returns size bytes of wired memory in the kernel map, zeroing the memory if the zeroit argument is non-zero.

uvm_km_kmemalloc() 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 */

UVM_KMF_NOWAIT 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.

uvm_km_valloc() and uvm_km_valloc_wait() 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.

ALLOCATION OF PHYSICAL MEMORY

struct vm_page * uvm_pagealloc(struct uvm_object *uobj, voff_t off, struct vm_anon *anon) 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)

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 off and uobj can be non NULL. Returns NULL when no page can be found.

uvm_pagerealloc() reallocates page pg to a new object newobj, at a new offset newoff.

uvm_pagefree() free's the physical page pg.

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. The nsegs and waitok arguments are currently ignored.

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

uvm_page_physload() loads physical memory segments into VM space. It must be called at system boot time to setup 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.

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 non-zero, which causes the min argument specify the beginning of the address in thes 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. If pageable is non-zero, entries in the map may be paged out.

uvm_km_free() and uvm_km_free_wakeup() free size bytes of memory in the kernal map, starting at address addr. uvm_km_free_wakeup() calls thread_wakeup() on the map before unlocking the map.

PROCESSES

void uvm_pageout() void uvm_scheduler() void uvm_swapin(struct proc *p)

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 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) boolean_t uvm_useracc(caddr_t addr, size_t len, int rw) void uvm_vslock(struct proc *p, caddr_t addr, size_t len) 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) 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 */

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() and uvm_useracc() check the access at address addr to addr + len for rw access, in the kernel address space, and the current process' address space respectively.

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_sysctl() 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 access 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 scaned 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 */

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 changed to allowed it to fail in low memory conditions.

uvm_grow() increases the stack segment of process p to include sp.

uvm_coredump() generates a coredump on vnode vp for process p with credentials cred and core header description in chdr.

STANDARD UVM RETURN VALUES
This section documents the standard return values that callers of UVM functions can expect. They are derived from the Mach VM values of the same function. The full list of values can be seen below.

    #define KERN_SUCCESS            0
    #define KERN_INVALID_ADDRESS    1
    #define KERN_PROTECTION_FAILURE 2
    #define KERN_NO_SPACE           3
    #define KERN_INVALID_ARGUMENT   4
    #define KERN_FAILURE            5
    #define KERN_RESOURCE_SHORTAGE  6
    #define KERN_NOT_RECEIVER       7
    #define KERN_NO_ACCESS          8
    #define KERN_PAGES_LOCKED       9

Note that KERN_NOT_RECEIVER and KERN_PAGES_LOCKED values are not actually returned by the UVM code.

NOTES
These functions are only available with options UVM.

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

The include file <vm/vm.h> will be deprecated when then Mach VM system is obsoleted. All structure and types whose names begin with ``vm_'' will be renamed to ``uvm_''.

The pmap(9) manual page is not yet written.

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 i386 machine-depenent layer includes several ideas from FreeBSD. UVM's new anonymous memory system is based on the anonymous memory system found in SunOS4 VM (as described in papers by published Sun Microsystems, Inc.). UVM also includes a number of feature new to BSD including page loanout, map entry passing, simplified copy-on-write, and clustered anonymous memory pageout. UVM will be further documented in August 1998 in a dissertation by Charles D. Cranor.

UVM appeared in NetBSD 1.4.

AUTHOR
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 involed 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.

SEE ALSO
getloadavg(3), kvm(3), sysctl(3), ddb(4), options(4)