64910ddbff
As of r234483, vnode deactivation causes non-VPO_NOSYNC pages to be laundered. This behaviour has two problems: 1. Dirty VPO_NOSYNC pages must be laundered before the vnode can be reclaimed, and this work may be unfairly deferred to the vnlru process or an unrelated application when the system is under vnode pressure. 2. Deactivation of a vnode with dirty VPO_NOSYNC pages requires a scan of the corresponding VM object's memq for non-VPO_NOSYNC dirty pages; if the laundry thread needs to launder pages from an unreferenced such vnode, it will reactivate and deactivate the vnode with each laundering, potentially resulting in a large number of expensive scans. Therefore, ensure that all dirty pages are laundered upon deactivation, i.e., when all maps of the vnode are removed and all references are released. Reviewed by: alc, kib MFC after: 1 month Differential Revision: https://reviews.freebsd.org/D8641
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453 lines
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.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
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.\" $FreeBSD$
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.\"
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.Dd November 25, 2016
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.Dt MMAP 2
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.Os
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.Sh NAME
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.Nm mmap
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.Nd allocate memory, or map files or devices into memory
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/mman.h
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.Ft void *
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.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
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.Sh DESCRIPTION
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The
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.Fn mmap
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system call causes the pages starting at
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.Fa addr
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and continuing for at most
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.Fa len
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bytes to be mapped from the object described by
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.Fa fd ,
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starting at byte offset
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.Fa offset .
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If
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.Fa len
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is not a multiple of the pagesize, the mapped region may extend past the
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specified range.
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Any such extension beyond the end of the mapped object will be zero-filled.
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.Pp
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If
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.Fa addr
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is non-zero, it is used as a hint to the system.
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(As a convenience to the system, the actual address of the region may differ
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from the address supplied.)
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If
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.Fa addr
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is zero, an address will be selected by the system.
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The actual starting address of the region is returned.
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A successful
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.Fa mmap
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deletes any previous mapping in the allocated address range.
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.Pp
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The protections (region accessibility) are specified in the
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.Fa prot
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argument by
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.Em or Ns 'ing
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the following values:
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.Pp
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.Bl -tag -width PROT_WRITE -compact
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.It Dv PROT_NONE
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Pages may not be accessed.
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.It Dv PROT_READ
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Pages may be read.
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.It Dv PROT_WRITE
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Pages may be written.
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.It Dv PROT_EXEC
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Pages may be executed.
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.El
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.Pp
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The
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.Fa flags
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argument specifies the type of the mapped object, mapping options and
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whether modifications made to the mapped copy of the page are private
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to the process or are to be shared with other references.
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Sharing, mapping type and options are specified in the
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.Fa flags
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argument by
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.Em or Ns 'ing
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the following values:
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.Bl -tag -width MAP_PREFAULT_READ
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.It Dv MAP_32BIT
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Request a region in the first 2GB of the current process's address space.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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This flag is only available on 64-bit platforms.
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.It Dv MAP_ALIGNED Ns Pq Fa n
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Align the region on a requested boundary.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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The
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.Fa n
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argument specifies the binary logarithm of the desired alignment.
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.It Dv MAP_ALIGNED_SUPER
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Align the region to maximize the potential use of large
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.Pq Dq super
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pages.
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If a suitable region cannot be found,
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.Fn mmap
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will fail.
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The system will choose a suitable page size based on the size of
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mapping.
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The page size used as well as the alignment of the region may both be
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affected by properties of the file being mapped.
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In particular,
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the physical address of existing pages of a file may require a specific
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alignment.
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The region is not guaranteed to be aligned on any specific boundary.
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.It Dv MAP_ANON
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Map anonymous memory not associated with any specific file.
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The file descriptor used for creating
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.Dv MAP_ANON
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must be \-1.
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The
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.Fa offset
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argument must be 0.
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.\".It Dv MAP_FILE
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.\"Mapped from a regular file or character-special device memory.
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.It Dv MAP_ANONYMOUS
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This flag is identical to
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.Dv MAP_ANON
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and is provided for compatibility.
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.It Dv MAP_EXCL
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This flag can only be used in combination with
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.Dv MAP_FIXED .
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Please see the definition of
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.Dv MAP_FIXED
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for the description of its effect.
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.It Dv MAP_FIXED
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Do not permit the system to select a different address than the one
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specified.
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If the specified address cannot be used,
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.Fn mmap
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will fail.
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If
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.Dv MAP_FIXED
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is specified,
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.Fa addr
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must be a multiple of the pagesize.
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If
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.Dv MAP_EXCL
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is not specified, a successful
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.Dv MAP_FIXED
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request replaces any previous mappings for the process'
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pages in the range from
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.Fa addr
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to
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.Fa addr
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+
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.Fa len .
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In contrast, if
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.Dv MAP_EXCL
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is specified, the request will fail if a mapping
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already exists within the range.
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.It Dv MAP_HASSEMAPHORE
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Notify the kernel that the region may contain semaphores and that special
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handling may be necessary.
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.It Dv MAP_NOCORE
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Region is not included in a core file.
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.It Dv MAP_NOSYNC
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Causes data dirtied via this VM map to be flushed to physical media
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only when necessary (usually by the pager) rather than gratuitously.
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Typically this prevents the update daemons from flushing pages dirtied
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through such maps and thus allows efficient sharing of memory across
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unassociated processes using a file-backed shared memory map.
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Without
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this option any VM pages you dirty may be flushed to disk every so often
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(every 30-60 seconds usually) which can create performance problems if you
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do not need that to occur (such as when you are using shared file-backed
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mmap regions for IPC purposes).
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Dirty data will be flushed automatically when all mappings of an object are
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removed and all descriptors referencing the object are closed.
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Note that VM/file system coherency is
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maintained whether you use
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.Dv MAP_NOSYNC
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or not.
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This option is not portable
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across
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.Ux
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platforms (yet), though some may implement the same behavior
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by default.
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.Pp
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.Em WARNING !
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Extending a file with
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.Xr ftruncate 2 ,
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thus creating a big hole, and then filling the hole by modifying a shared
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.Fn mmap
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can lead to severe file fragmentation.
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In order to avoid such fragmentation you should always pre-allocate the
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file's backing store by
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.Fn write Ns ing
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zero's into the newly extended area prior to modifying the area via your
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.Fn mmap .
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The fragmentation problem is especially sensitive to
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.Dv MAP_NOSYNC
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pages, because pages may be flushed to disk in a totally random order.
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.Pp
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The same applies when using
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.Dv MAP_NOSYNC
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to implement a file-based shared memory store.
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It is recommended that you create the backing store by
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.Fn write Ns ing
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zero's to the backing file rather than
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.Fn ftruncate Ns ing
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it.
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You can test file fragmentation by observing the KB/t (kilobytes per
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transfer) results from an
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.Dq Li iostat 1
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while reading a large file sequentially, e.g.,\& using
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.Dq Li dd if=filename of=/dev/null bs=32k .
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.Pp
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The
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.Xr fsync 2
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system call will flush all dirty data and metadata associated with a file,
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including dirty NOSYNC VM data, to physical media.
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The
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.Xr sync 8
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command and
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.Xr sync 2
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system call generally do not flush dirty NOSYNC VM data.
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The
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.Xr msync 2
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system call is usually not needed since
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.Bx
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implements a coherent file system buffer cache.
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However, it may be
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used to associate dirty VM pages with file system buffers and thus cause
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them to be flushed to physical media sooner rather than later.
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.It Dv MAP_PREFAULT_READ
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Immediately update the calling process's lowest-level virtual address
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translation structures, such as its page table, so that every memory
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resident page within the region is mapped for read access.
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Ordinarily these structures are updated lazily.
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The effect of this option is to eliminate any soft faults that would
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otherwise occur on the initial read accesses to the region.
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Although this option does not preclude
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.Fa prot
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from including
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.Dv PROT_WRITE ,
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it does not eliminate soft faults on the initial write accesses to the
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region.
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.It Dv MAP_PRIVATE
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Modifications are private.
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.It Dv MAP_SHARED
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Modifications are shared.
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.It Dv MAP_STACK
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.Dv MAP_STACK
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implies
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.Dv MAP_ANON ,
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and
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.Fa offset
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of 0.
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The
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.Fa fd
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argument
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must be -1 and
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.Fa prot
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must include at least
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.Dv PROT_READ
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and
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.Dv PROT_WRITE .
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This option creates
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a memory region that grows to at most
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.Fa len
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bytes in size, starting from the stack top and growing down.
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The
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stack top is the starting address returned by the call, plus
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.Fa len
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bytes.
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The bottom of the stack at maximum growth is the starting
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address returned by the call.
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.El
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.Pp
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The
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.Xr close 2
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system call does not unmap pages, see
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.Xr munmap 2
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for further information.
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.Sh NOTES
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Although this implementation does not impose any alignment restrictions on
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the
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.Fa offset
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argument, a portable program must only use page-aligned values.
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.Pp
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Large page mappings require that the pages backing an object be
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aligned in matching blocks in both the virtual address space and RAM.
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The system will automatically attempt to use large page mappings when
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mapping an object that is already backed by large pages in RAM by
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aligning the mapping request in the virtual address space to match the
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alignment of the large physical pages.
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The system may also use large page mappings when mapping portions of an
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object that are not yet backed by pages in RAM.
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The
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.Dv MAP_ALIGNED_SUPER
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flag is an optimization that will align the mapping request to the
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size of a large page similar to
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.Dv MAP_ALIGNED ,
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except that the system will override this alignment if an object already
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uses large pages so that the mapping will be consistent with the existing
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large pages.
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This flag is mostly useful for maximizing the use of large pages on the
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first mapping of objects that do not yet have pages present in RAM.
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn mmap
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returns a pointer to the mapped region.
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Otherwise, a value of
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.Dv MAP_FAILED
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is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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The
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.Fn mmap
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system call
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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The flag
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.Dv PROT_READ
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was specified as part of the
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.Fa prot
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argument and
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.Fa fd
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was not open for reading.
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The flags
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.Dv MAP_SHARED
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and
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.Dv PROT_WRITE
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were specified as part of the
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.Fa flags
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and
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.Fa prot
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argument and
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.Fa fd
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was not open for writing.
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.It Bq Er EBADF
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The
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.Fa fd
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argument
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is not a valid open file descriptor.
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.It Bq Er EINVAL
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An invalid value was passed in the
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.Fa prot
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argument.
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.It Bq Er EINVAL
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An undefined option was set in the
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.Fa flags
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argument.
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.It Bq Er EINVAL
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Both
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.Dv MAP_PRIVATE
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and
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.Dv MAP_SHARED
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were specified.
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.It Bq Er EINVAL
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None of
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.Dv MAP_ANON ,
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.Dv MAP_PRIVATE ,
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.Dv MAP_SHARED ,
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or
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.Dv MAP_STACK
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was specified.
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At least one of these flags must be included.
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.It Bq Er EINVAL
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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argument was not page aligned, or part of the desired address space
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resides out of the valid address space for a user process.
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.It Bq Er EINVAL
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Both
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.Dv MAP_FIXED
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and
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.Dv MAP_32BIT
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were specified and part of the desired address space resides outside
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of the first 2GB of user address space.
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.It Bq Er EINVAL
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The
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.Fa len
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argument
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was equal to zero.
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.It Bq Er EINVAL
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.Dv MAP_ALIGNED
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was specified and the desired alignment was either larger than the
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virtual address size of the machine or smaller than a page.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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was specified and the
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.Fa fd
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argument was not -1.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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was specified and the
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.Fa offset
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argument was not 0.
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.It Bq Er EINVAL
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Both
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.Dv MAP_FIXED
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and
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.Dv MAP_EXCL
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were specified, but the requested region is already used by a mapping.
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.It Bq Er EINVAL
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.Dv MAP_EXCL
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was specified, but
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.Dv MAP_FIXED
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was not.
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.It Bq Er ENODEV
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.Dv MAP_ANON
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has not been specified and
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.Fa fd
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did not reference a regular or character special file.
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.It Bq Er ENOMEM
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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argument was not available.
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.Dv MAP_ANON
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was specified and insufficient memory was available.
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.El
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.Sh SEE ALSO
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.Xr madvise 2 ,
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.Xr mincore 2 ,
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.Xr minherit 2 ,
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.Xr mlock 2 ,
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.Xr mprotect 2 ,
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.Xr msync 2 ,
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.Xr munlock 2 ,
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.Xr munmap 2 ,
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.Xr getpagesize 3 ,
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.Xr getpagesizes 3
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