VM_PAGE_BUSY(9)        FreeBSD Kernel Developer's Manual       VM_PAGE_BUSY(9)

NAME
     vm_page_busied, vm_page_busy_downgrade, vm_page_busy_sleep,
     vm_page_sbusied, vm_page_sunbusy, vm_page_trysbusy, vm_page_tryxbusy,
     vm_page_xbusied, 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);

     bool
     vm_page_busy_sleep(vm_page_t m, const char *msg, int allocflags);

     int
     vm_page_sbusied(vm_page_t m);

     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_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() checks the busy state of the page m and puts the
     invoking thread to sleep if the page is busy.  The VM object for the page
     must be locked.  The allocflags parameter must be either 0, in which case
     the function will sleep if the page is busied, or VM_ALLOC_IGN_SBUSY, in
     which case the function will sleep only if the page is exclusively
     busied.  A return value of true indicates that the invoking thread was
     put to sleep and that the object was unlocked.  A return value of false
     indicates that the invoking thread did not sleep and the object remains
     locked.  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_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_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)

FreeBSD 13.1-RELEASE-p6        November 11, 2021       FreeBSD 13.1-RELEASE-p6