man/man2/memcntl.2

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.Dd March 13, 2022
.Dt MEMCNTL 2
.Os
.Sh NAME
.Nm memcntl
.Nd memory management control
.Sh SYNOPSIS
.In sys/types.h
.In sys/mman.h
.Ft int
.Fo memcntl
.Fa "void *addr"
.Fa "size_t len"
.Fa "int cmd"
.Fa "void *arg"
.Fa "int attr"
.Fa "int mask"
.Fc
.Sh DESCRIPTION
The
.Fn memcntl
function allows the calling process to apply a variety of control operations
over the address space identified by the mappings established for the address
range [
.Fa addr ,
.Fa addr
+
.Fa len
).
.Pp
The
.Fa addr
argument must be a multiple of the pagesize as returned by
.Xr sysconf 3C .
The scope of the control operations can be further defined with additional
selection criteria
.Pq in the form of attributes
according to the bit pattern contained in
.Fa attr .
.Pp
The following attributes specify page mapping selection criteria:
.Bl -tag -width Ds -offset indent
.It Dv MEMCNTL_SHARED
Page is mapped shared.
.It Dv MEMCNTL_PRIVATE
Page is mapped private.
.El
.Pp
The following attributes specify page protection selection criteria.
The selection criteria are constructed by a bitwise OR operation on the
attribute bits and must match exactly.
.Bl -tag -width Ds -offset indent
.It Dv PROT_READ
Page can be read.
.It Dv PROT_WRITE
Page can be written.
.It Dv PROT_EXEC
Page can be executed.
.El
.Pp
The following criteria may also be specified:
.Bl -tag -width Ds -offset indent
.It Dv PROC_TEXT
Process text.
.It Dv PROC_DATA
Process data.
.El
.Pp
The
.Dv PROC_TEXT
attribute specifies all privately mapped segments with read and execute
permission, and the
.Dv PROC_DATA
attribute specifies all privately mapped segments with write permission.
.Pp
Selection criteria can be used to describe various abstract memory objects
within the address space on which to operate.
If an operation shall not be constrained by the selection criteria,
.Fa attr
must have the value
.Sy 0 .
.Pp
The operation to be performed is identified by the argument
.Fa cmd .
The symbolic names for the operations are defined in
.In sys/mman.h
as follows:
.Bl -tag -width Ds -offset indent
.It Dv MC_LOCK
Lock in memory all pages in the range with attributes
.Fa attr .
A given page may be locked multiple times through different mappings; however,
within a given mapping, page locks do not nest.
Multiple lock operations on the same address in the same process will all be
removed with a single unlock operation.
A page locked in one process and mapped in another
.Pq or visible through a different mapping in the locking process
is locked in memory as long as the locking process does neither an implicit nor
explicit unlock operation.
If a locked mapping is removed, or a page is deleted through file removal or
truncation, an unlock operation is implicitly performed.
If a writable
.Dv MAP_PRIVATE
page in the address range is changed, the lock will be transferred to the
private page.
.Pp
The
.Fa arg
argument is not used, but must be
.Dv NULL
to ensure compatibility with potential future enhancements.
.It Dv MC_LOCKAS
Lock in memory all pages mapped by the address space with attributes
.Fa attr .
The
.Fa addr
and
.Fa len
arguments are not used, but must be
.Dv NULL
and
.Sy 0
respectively, to ensure compatibility with potential future enhancements.
The
.Fa arg
argument is a bit pattern built from the flags:
.Bl -tag -width Ds
.It Dv MCL_CURRENT
Lock current mappings.
.It Dv MCL_FUTURE
Lock future mappings.
.El
.Pp
The value of
.Fa arg
determines whether the pages to be locked are those currently mapped by the
address space, those that will be mapped in the future, or both.
If
.Dv MCL_FUTURE
is specified, then all mappings subsequently added to the address space will be
locked, provided sufficient memory is available.
.It Dv MC_SYNC
Write to their backing storage locations all modified pages in the range with
attributes
.Fa attr .
Optionally, invalidate cache copies.
The backing storage for a modified
.Dv MAP_SHARED
mapping is the file the page is mapped to; the backing storage for a modified
.Dv MAP_PRIVATE
mapping is its swap area.
The
.Fa arg
argument is a bit pattern built from the flags used to control the behavior of
the operation:
.Bl -tag -width Ds
.It Dv MS_ASYNC
Perform asynchronous writes.
The function returns immediately once all write operations are scheduled
.Po unless
.Dv MS_SYNC
is also specified
.Pc .
.It Dv MS_SYNC
Perform synchronous writes.
The function will not return until all write operations are completed.
.It Dv MS_INVALIDATE
Invalidate all cached copies of data in memory, so that further references to
the pages will be obtained by the system from their backing storage locations.
This operation should be used by applications that require a memory object to be
in a known state.
.El
.It Dv MC_UNLOCK
Unlock all pages in the range with attributes
.Fa attr .
The
.Fa arg
argument is not used, but must be
.Dv NULL
to ensure compatibility with potential future enhancements.
.It Dv MC_UNLOCKAS
Remove address space memory locks and locks on all pages in the address space
with attributes
.Fa attr .
The
.Fa addr ,
.Fa len ,
and
.Fa arg
arguments are not used, but must be
.Dv NULL ,
0, and
.Dv NULL ,
respectively, to ensure compatibility with potential future enhancements.
.It Dv MC_HAT_ADVISE
Advise system how a region of user-mapped memory will be accessed.
The
.Fa arg
argument is interpreted as a
.Dq Vt struct memcntl_mha * .
The following members are defined in a
.Vt struct memcntl_mha :
.Bd -literal -offset indent
uint_t mha_cmd;
uint_t mha_flags;
size_t mha_pagesize;
.Ed
.Pp
The accepted values for
.Vt mha_cmd
are:
.Bd -literal -offset indent
MHA_MAPSIZE_VA
MHA_MAPSIZE_STACK
MHA_MAPSIZE_BSSBRK
.Ed
.Pp
The
.Fa mha_flags
member is reserved for future use and must always be set to 0.
The
.Fa mha_pagesize
member must be a valid size as obtained from
.Xr getpagesizes 3C
or the constant value 0 to allow the system to choose an appropriate hardware
address translation mapping size.
.Pp
.Dv MHA_MAPSIZE_VA
sets the preferred hardware address translation mapping size of the region of
memory from
.Fa addr
to
.Fa addr
+
.Fa len .
Both
.Fa addr
and
.Fa len
must be aligned to an
.Fa mha_pagesize
boundary.
The entire virtual address region from
.Fa addr
to
.Fa addr
+
.Fa len
must not have any holes.
Permissions within each
.Fa mha_pagesize Ns -aligned
portion of the region must be consistent.
When a size of 0 is specified, the system selects an appropriate size based on
the size and alignment of the memory region, type of processor, and other
considerations.
.Pp
.Dv MHA_MAPSIZE_STACK
sets the preferred hardware address translation mapping size of the process main
thread stack segment.
The
.Fa addr
and
.Fa len
arguments must be
.Dv NULL
and 0, respectively.
.Pp
.Dv MHA_MAPSIZE_BSSBRK
sets the preferred hardware address translation mapping size of the process
heap.
The
.Fa addr
and
.Fa len
arguments must be
.Dv NULL
and 0, respectively.
See the NOTES section of the
.Xr ppgsz 1
manual page for additional information on process heap alignment.
.Pp
The
.Fa attr
argument must be 0 for all
.Dv MC_HAT_ADVISE
operations.
.El
.Pp
The
.Fa mask
argument must be 0; it is reserved for future use.
.Pp
Locks established with the lock operations are not inherited by a child process
after
.Xr fork 2 .
The
.Fn memcntl
function fails if it attempts to lock more memory than a system-specific limit.
.Pp
Due to the potential impact on system resources, the operations
.Dv MC_LOCKAS ,
.Dv MC_LOCK ,
.Dv MC_UNLOCKAS ,
and
.Dv MC_UNLOCK
are restricted to privileged processes.
.Sh USAGE
The
.Fn memcntl
function subsumes the operations of
.Xr plock 3C .
.Pp
.Dv MC_HAT_ADVISE
is intended to improve performance of applications that use large amounts of
memory on processors that support multiple hardware address translation mapping
sizes; however, it should be used with care.
Not all processors support all sizes with equal efficiency.
Use of larger sizes may also introduce extra overhead that could reduce
performance or available memory.
Using large sizes for one application may reduce available resources for other
applications and result in slower system wide performance.
.Sh RETURN VALUES
Upon successful completion,
.Fn memcntl
returns
.Sy 0 .
Otherwise, it returns
.Sy -1
and sets
.Va errno
to indicate an error.
.Sh ERRORS
The
.Fn memcntl
function will fail if:
.Bl -tag -width Er
.It Er EAGAIN
When the selection criteria match, some or all of the memory identified by the
operation could not be locked when
.Dv MC_LOCK
or
.Dv MC_LOCKAS
was specified, some or all mappings in the address range
.Pf [ Fa addr ,
.Fa addr
+
.Fa len Ns
) are locked for I/O when
.Dv MC_HAT_ADVISE
was specified, or the system has insufficient resources when
.Dv MC_HAT_ADVISE
was specified.
.Pp
The
.Fa cmd
is
.Dv MC_LOCK
or
.Dv MC_LOCKAS
and locking the memory identified by this operation would exceed a limit or
resource control on locked memory.
.It Er EBUSY
When the selection criteria match, some or all of the addresses in the range
.Pf [ Fa addr ,
.Fa addr
+
.Fa len Ns
) are locked and
.Dv MC_SYNC with the
.Dv MS_INVALIDATE
option was specified.
.It Er EINVAL
The
.Fa addr
argument specifies invalid selection criteria or is not a multiple of the page
size as returned by
.Xr sysconf 3C ;
the
.Fa addr
and/or
.Fa len
argument does not have the value 0 when
.Dv MC_LOCKAS
or
.Dv MC_UNLOCKAS
is specified; the
.Fa arg
argument is not valid for the function specified;
.Fa mha_pagesize
or
.Fa mha_cmd
is invalid; or
.Dv MC_HAT_ADVISE
is specified and not all pages in the specified region have the same access
permissions within the given size boundaries.
.It Er ENOMEM
When the selection criteria match, some or all of the addresses in the range
.Pf [ Fa addr ,
.Fa addr
+
.Fa len Ns
) are invalid for the address space of a process or specify one or more pages
which are not mapped.
.It Er EPERM
The
.Brq Dv PRIV_PROC_LOCK_MEMORY
privilege is not asserted in the effective set of the calling process and
.Dv MC_LOCK ,
.Dv MC_LOCKAS ,
.Dv MC_UNLOCK ,
or
.Dv MC_UNLOCKAS
was specified.
.El
.Sh MT-LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr ppgsz 1 ,
.Xr fork 2 ,
.Xr mmap 2 ,
.Xr mprotect 2 ,
.Xr getpagesizes 3C ,
.Xr mlock 3C ,
.Xr mlockall 3C ,
.Xr msync 3C ,
.Xr plock 3C ,
.Xr sysconf 3C ,
.Xr attributes 7 ,
.Xr privileges 7