NAME

     vm_page_busied, vm_page_busy_downgrade, vm_page_busy_sleep, vm_page_sbusied, vm_page_sbusy,
     vm_page_sleep_if_busy, vm_page_sunbusy, vm_page_trysbusy, vm_page_tryxbusy, vm_page_xbusied,
     vm_page_xbusy, vm_page_xunbusy, vm_page_assert_sbusied, vm_page_assert_unbusied,
     vm_page_assert_xbusied — protect page identity changes and page content references

SYNOPSIS

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

     int
     vm_page_busied(vm_page_t m);

     void
     vm_page_busy_downgrade(vm_page_t m);

     void
     vm_page_busy_sleep(vm_page_t m, const char *msg);

     int
     vm_page_sbusied(vm_page_t m);

     void
     vm_page_sbusy(vm_page_t m);

     int
     vm_page_sleep_if_busy(vm_page_t m, const char *msg);

     void
     vm_page_sunbusy(vm_page_t m);

     int
     vm_page_trysbusy(vm_page_t m);

     int
     vm_page_tryxbusy(vm_page_t m);

     int
     vm_page_xbusied(vm_page_t m);

     void
     vm_page_xbusy(vm_page_t m);

     void
     vm_page_xunbusy(vm_page_t m);

     options INVARIANTS
     options INVARIANT_SUPPORT

     void
     vm_page_assert_sbusied(vm_page_t m);

     void
     vm_page_assert_unbusied(vm_page_t m);

     void
     vm_page_assert_xbusied(vm_page_t m);

DESCRIPTION

     Page identity is usually protected by higher level locks like vm_object locks and vm page
     locks.  However, sometimes it is not possible to hold such locks for the time necessary to
     complete the identity change.  In such case the page can be exclusively busied by a thread
     which needs to own the identity for a certain amount of time.

     In other situations, threads do not need to change the identity of the page but they want to
     prevent other threads from changing the identity themselves.  For example, when a thread
     wants to access or update page contents without a lock held the page is shared busied.

     Before busying a page the vm_object lock must be held.  The same rule applies when a page is
     unbusied.  This makes the vm_object lock a real busy interlock.

     The vm_page_busied() function returns non-zero if the current thread busied m in either
     exclusive or shared mode.  Returns zero otherwise.

     The vm_page_busy_downgrade() function must be used to downgrade m from an exclusive busy
     state to a shared busy state.

     The vm_page_busy_sleep() function puts the invoking thread to sleep using the appropriate
     waitchannels for the busy mechanism.  The parameter msg is a string describing the sleep
     condition for userland tools.

     The vm_page_busied() function returns non-zero if the current thread busied m in shared
     mode.  Returns zero otherwise.

     The vm_page_sbusy() function shared busies m.

     The vm_page_sleep_if_busy() function puts the invoking thread to sleep, using the
     appropriate waitchannels for the busy mechanism, if m.  is busied in either exclusive or
     shared mode.  If the invoking thread slept a non-zero value is returned, otherwise 0 is
     returned.  The parameter msg is a string describing the sleep condition for userland tools.

     The vm_page_sunbusy() function shared unbusies m.

     The vm_page_trysbusy() attempts to shared busy m.  If the operation cannot immediately
     succeed vm_page_trysbusy() returns 0, otherwise a non-zero value is returned.

     The vm_page_tryxbusy() attempts to exclusive busy m.  If the operation cannot immediately
     succeed vm_page_tryxbusy() returns 0, otherwise a non-zero value is returned.

     The vm_page_xbusied() function returns non-zero if the current thread busied m in exclusive
     mode.  Returns zero otherwise.

     The vm_page_xbusy() function exclusive busies m.

     The vm_page_xunbusy() function exclusive unbusies m.  Assertions on the busy state allow
     kernels compiled with options INVARIANTS and options INVARIANT_SUPPORT to panic if they are
     not respected.

     The vm_page_assert_sbusied() function panics if m is not shared busied.

     The vm_page_assert_unbusied() function panics if m is not unbusied.

     The vm_page_assert_xbusied() function panics if m is not exclusive busied.

SEE ALSO

     vm_page_aflag(9), vm_page_alloc(9), vm_page_deactivate(9), vm_page_free(9), vm_page_grab(9),
     vm_page_insert(9), vm_page_lookup(9), vm_page_rename(9), VOP_GETPAGES(9)