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.Dd August 07, 2013
.Dt VM_PAGE_BUSY 9
.Os
.Sh NAME
.Nm vm_page_busied ,
.Nm vm_page_busy_downgrade ,
.Nm vm_page_busy_sleep ,
.Nm vm_page_sbusied ,
.Nm vm_page_sbusy ,
.Nm vm_page_sleep_if_busy ,
.Nm vm_page_sunbusy ,
.Nm vm_page_trysbusy ,
.Nm vm_page_tryxbusy ,
.Nm vm_page_xbusied ,
.Nm vm_page_xbusy ,
.Nm vm_page_xunbusy ,
.Nm vm_page_assert_sbusied ,
.Nm vm_page_assert_unbusied ,
.Nm vm_page_assert_xbusied
.Nd protect page identity changes and page content references
.Sh SYNOPSIS
.In sys/param.h
.In vm/vm.h
.In vm/vm_page.h
.Ft int
.Fn vm_page_busied "vm_page_t m"
.Ft void
.Fn vm_page_busy_downgrade "vm_page_t m"
.Ft void
.Fn vm_page_busy_sleep "vm_page_t m" "const char *msg"
.Ft int
.Fn vm_page_sbusied "vm_page_t m"
.Ft void
.Fn vm_page_sbusy "vm_page_t m"
.Ft int
.Fn vm_page_sleep_if_busy "vm_page_t m" "const char *msg"
.Ft void
.Fn vm_page_sunbusy "vm_page_t m"
.Ft int
.Fn vm_page_trysbusy "vm_page_t m"
.Ft int
.Fn vm_page_tryxbusy "vm_page_t m"
.Ft int
.Fn vm_page_xbusied "vm_page_t m"
.Ft void
.Fn vm_page_xbusy "vm_page_t m"
.Ft void
.Fn vm_page_xunbusy "vm_page_t m"
.Pp
.Cd "options INVARIANTS"
.Cd "options INVARIANT_SUPPORT"
.Ft void
.Fn vm_page_assert_sbusied "vm_page_t m"
.Ft void
.Fn vm_page_assert_unbusied "vm_page_t m"
.Ft void
.Fn vm_page_assert_xbusied "vm_page_t m"
.Sh 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.
.Pp
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.
.Pp
Before busing 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.
.Pp
The
.Fn vm_page_busied
function returns non-zero if the current thread busied
.Fa m
in either exclusive or shared mode.
Returns zero otherwise.
.Pp
The
.Fn vm_page_busy_downgrade
function must be used to downgrade
.Fa m
from an exclusive busy state to a shared busy state.
.Pp
The
.Fn vm_page_busy_sleep
function puts the invoking thread to sleep using the appropriate
waitchannels for the busy mechanism.
The parameter
.Fa msg
is a string describing the sleep condition for userland tools.
.Pp
The
.Fn vm_page_busied
function returns non-zero if the current thread busied
.Fa m
in shared mode.
Returns zero otherwise.
.Pp
The
.Fn vm_page_sbusy
function shared busies
.Fa m .
.Pp
The
.Fn vm_page_sleep_if_busy
function puts the invoking thread to sleep, using the appropriate
waitchannels for the busy mechanism, if
.Fa 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
.Fa msg
is a string describing the sleep condition for userland tools.
.Pp
The
.Fn vm_page_sunbusy
function shared unbusies
.Fa m .
.Pp
The
.Fn vm_page_trysbusy
attempts to shared busy
.Fa m .
If the operation cannot immediately succeed
.Fn vm_page_trysbusy
returns 0, otherwise a non-zero value is returned.
.Pp
The
.Fn vm_page_tryxbusy
attempts to exclusive busy
.Fa m .
If the operation cannot immediately succeed
.Fn vm_page_tryxbusy
returns 0, otherwise a non-zero value is returned.
.Pp
The
.Fn vm_page_xbusied
function returns non-zero if the current thread busied
.Fa m
in exclusive mode.
Returns zero otherwise.
.Pp
The
.Fn vm_page_xbusy
function exclusive busies
.Fa m .
.Pp
The
.Fn vm_page_xunbusy
function exclusive unbusies
.Fa m .
Assertions on the busy state allow kernels compiled with
.Cd "options INVARIANTS"
and
.Cd "options INVARIANT_SUPPORT"
to panic if they are not respected.
.Pp
The
.Fn vm_page_assert_sbusied
function panics if
.Fa m
is not shared busied.
.Pp
The
.Fn vm_page_assert_unbusied
function panics if
.Fa m
is not unbusied.
.Pp
The
.Fn vm_page_assert_xbusied
function panics if
.Fa m
is not exclusive busied.
.Sh SEE ALSO
.Xr VOP_GETPAGES 9 ,
.Xr vm_page_aflag 9 ,
.Xr vm_page_alloc 9 ,
.Xr vm_page_deactivate 9 ,
.Xr vm_page_free 9 ,
.Xr vm_page_grab 9 ,
.Xr vm_page_insert 9 ,
.Xr vm_page_lookup 9 ,
.Xr vm_page_rename 9