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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_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)

FreeBSD 11.1-RELEASE-p4         August 7, 2013         FreeBSD 11.1-RELEASE-p4
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