diff mbox

[xfsprogs,1/2] xfs_io: add MAP_SYNC support to mmap()

Message ID 20171117202524.24696-2-ross.zwisler@linux.intel.com (mailing list archive)
State New, archived
Headers show

Commit Message

Ross Zwisler Nov. 17, 2017, 8:25 p.m. UTC 
Add support for a new -S flag to xfs_io's mmap command.  This opens the
mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
standard MAP_SHARED flag.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Suggested-by: Dave Chinner <david@fromorbit.com>
---
 configure.ac          |  1 +
 include/builddefs.in  |  1 +
 include/linux.h       |  5 +++++
 io/io.h               |  1 +
 io/mmap.c             | 19 ++++++++++++++-----
 m4/package_libcdev.m4 | 16 ++++++++++++++++
 man/man8/xfs_io.8     |  6 +++++-
 7 files changed, 43 insertions(+), 6 deletions(-)

Comments

Dan Williams Nov. 17, 2017, 8:35 p.m. UTC | #1 
On Fri, Nov 17, 2017 at 12:25 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> Add support for a new -S flag to xfs_io's mmap command.  This opens the
> mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
> standard MAP_SHARED flag.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> ---
>  configure.ac          |  1 +
>  include/builddefs.in  |  1 +
>  include/linux.h       |  5 +++++
>  io/io.h               |  1 +
>  io/mmap.c             | 19 ++++++++++++++-----
>  m4/package_libcdev.m4 | 16 ++++++++++++++++
>  man/man8/xfs_io.8     |  6 +++++-
>  7 files changed, 43 insertions(+), 6 deletions(-)
>
> diff --git a/configure.ac b/configure.ac
> index cb23fb8..7153c9f 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -162,6 +162,7 @@ AC_HAVE_FSETXATTR
>  AC_HAVE_MREMAP
>  AC_NEED_INTERNAL_FSXATTR
>  AC_HAVE_GETFSMAP
> +AC_HAVE_MAP_SYNC
>
>  if test "$enable_blkid" = yes; then
>  AC_HAVE_BLKID_TOPO
> diff --git a/include/builddefs.in b/include/builddefs.in
> index 1d454b6..78b71fe 100644
> --- a/include/builddefs.in
> +++ b/include/builddefs.in
> @@ -114,6 +114,7 @@ HAVE_FSETXATTR = @have_fsetxattr@
>  HAVE_MREMAP = @have_mremap@
>  NEED_INTERNAL_FSXATTR = @need_internal_fsxattr@
>  HAVE_GETFSMAP = @have_getfsmap@
> +HAVE_MAP_SYNC = @have_map_sync@
>
>  GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
>  #         -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-decl
> diff --git a/include/linux.h b/include/linux.h
> index 6ce344c..4ee03ed 100644
> --- a/include/linux.h
> +++ b/include/linux.h
> @@ -327,4 +327,9 @@ fsmap_advance(
>  #define HAVE_GETFSMAP
>  #endif /* HAVE_GETFSMAP */
>
> +#ifndef HAVE_MAP_SYNC
> +#define MAP_SYNC 0x80000

Hmm, this is the x86 specific value of MAP_SYNC. I think it would be
better to define MAP_SYNC to zero in this case and check for MAP_SYNC
== 0 at run time.
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Darrick J. Wong Nov. 17, 2017, 8:40 p.m. UTC | #2 
On Fri, Nov 17, 2017 at 01:25:23PM -0700, Ross Zwisler wrote:
> Add support for a new -S flag to xfs_io's mmap command.  This opens the
> mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
> standard MAP_SHARED flag.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> ---
>  configure.ac          |  1 +
>  include/builddefs.in  |  1 +
>  include/linux.h       |  5 +++++
>  io/io.h               |  1 +
>  io/mmap.c             | 19 ++++++++++++++-----
>  m4/package_libcdev.m4 | 16 ++++++++++++++++
>  man/man8/xfs_io.8     |  6 +++++-
>  7 files changed, 43 insertions(+), 6 deletions(-)
> 
> diff --git a/configure.ac b/configure.ac
> index cb23fb8..7153c9f 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -162,6 +162,7 @@ AC_HAVE_FSETXATTR
>  AC_HAVE_MREMAP
>  AC_NEED_INTERNAL_FSXATTR
>  AC_HAVE_GETFSMAP
> +AC_HAVE_MAP_SYNC
>  
>  if test "$enable_blkid" = yes; then
>  AC_HAVE_BLKID_TOPO
> diff --git a/include/builddefs.in b/include/builddefs.in
> index 1d454b6..78b71fe 100644
> --- a/include/builddefs.in
> +++ b/include/builddefs.in
> @@ -114,6 +114,7 @@ HAVE_FSETXATTR = @have_fsetxattr@
>  HAVE_MREMAP = @have_mremap@
>  NEED_INTERNAL_FSXATTR = @need_internal_fsxattr@
>  HAVE_GETFSMAP = @have_getfsmap@
> +HAVE_MAP_SYNC = @have_map_sync@
>  
>  GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
>  #	   -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-decl
> diff --git a/include/linux.h b/include/linux.h
> index 6ce344c..4ee03ed 100644
> --- a/include/linux.h
> +++ b/include/linux.h
> @@ -327,4 +327,9 @@ fsmap_advance(
>  #define HAVE_GETFSMAP
>  #endif /* HAVE_GETFSMAP */
>  
> +#ifndef HAVE_MAP_SYNC
> +#define MAP_SYNC 0x80000
> +#define MAP_SHARED_VALIDATE 0x3
> +#endif /* HAVE_MAP_SYNC */

Hmm, what's the point of ifndef/define if you have an configure.ac check?

> +
>  #endif	/* __XFS_LINUX_H__ */
> diff --git a/io/io.h b/io/io.h
> index 3862985..8b2753b 100644
> --- a/io/io.h
> +++ b/io/io.h
> @@ -65,6 +65,7 @@ typedef struct mmap_region {
>  	size_t		length;		/* length of mapping */
>  	off64_t		offset;		/* start offset into backing file */
>  	int		prot;		/* protection mode of the mapping */
> +	bool		map_sync;	/* is this a MAP_SYNC mapping? */
>  	char		*name;		/* name of backing file */
>  } mmap_region_t;
>  
> diff --git a/io/mmap.c b/io/mmap.c
> index 7a8150e..520b037 100644
> --- a/io/mmap.c
> +++ b/io/mmap.c
> @@ -42,7 +42,7 @@ print_mapping(
>  	int		index,
>  	int		braces)
>  {
> -	unsigned char	buffer[8] = { 0 };
> +	char		buffer[8] = { 0 };
>  	int		i;
>  
>  	static struct {
> @@ -57,6 +57,10 @@ print_mapping(
>  
>  	for (i = 0, p = pflags; p->prot != PROT_NONE; i++, p++)
>  		buffer[i] = (map->prot & p->prot) ? p->mode : '-';
> +
> +	if (map->map_sync)
> +		sprintf(&buffer[i], " S");

Does buffer need enlarging here?

> +
>  	printf("%c%03d%c 0x%lx - 0x%lx %s  %14s (%lld : %ld)\n",
>  		braces? '[' : ' ', index, braces? ']' : ' ',
>  		(unsigned long)map->addr,
> @@ -146,6 +150,7 @@ mmap_help(void)
>  " -r -- map with PROT_READ protection\n"
>  " -w -- map with PROT_WRITE protection\n"
>  " -x -- map with PROT_EXEC protection\n"
> +" -S -- map with MAP_SHARED_VALIDATE and MAP_SYNC flags\n"
>  " -s <size> -- first do mmap(size)/munmap(size), try to reserve some free space\n"
>  " If no protection mode is specified, all are used by default.\n"
>  "\n"));
> @@ -161,7 +166,7 @@ mmap_f(
>  	void		*address = NULL;
>  	char		*filename;
>  	size_t		blocksize, sectsize;
> -	int		c, prot = 0;
> +	int		c, prot = 0, flags = MAP_SHARED;
>  
>  	if (argc == 1) {
>  		if (mapping)
> @@ -184,7 +189,7 @@ mmap_f(
>  
>  	init_cvtnum(&blocksize, &sectsize);
>  
> -	while ((c = getopt(argc, argv, "rwxs:")) != EOF) {
> +	while ((c = getopt(argc, argv, "rwxSs:")) != EOF) {
>  		switch (c) {
>  		case 'r':
>  			prot |= PROT_READ;
> @@ -195,6 +200,9 @@ mmap_f(
>  		case 'x':
>  			prot |= PROT_EXEC;
>  			break;
> +		case 'S':
> +			flags = MAP_SHARED_VALIDATE | MAP_SYNC;
> +			break;
>  		case 's':
>  			length2 = cvtnum(blocksize, sectsize, optarg);
>  			break;
> @@ -238,7 +246,7 @@ mmap_f(
>  		               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>  		munmap(address, length2);
>  	}
> -	address = mmap(address, length, prot, MAP_SHARED, file->fd, offset);
> +	address = mmap(address, length, prot, flags, file->fd, offset);
>  	if (address == MAP_FAILED) {
>  		perror("mmap");
>  		free(filename);
> @@ -263,6 +271,7 @@ mmap_f(
>  	mapping->offset = offset;
>  	mapping->name = filename;
>  	mapping->prot = prot;
> +	mapping->map_sync = (flags == (MAP_SHARED_VALIDATE | MAP_SYNC));
>  	return 0;
>  }
>  
> @@ -676,7 +685,7 @@ mmap_init(void)
>  	mmap_cmd.argmax = -1;
>  	mmap_cmd.flags = CMD_NOMAP_OK | CMD_NOFILE_OK |
>  			 CMD_FOREIGN_OK | CMD_FLAG_ONESHOT;
> -	mmap_cmd.args = _("[N] | [-rwx] [-s size] [off len]");
> +	mmap_cmd.args = _("[N] | [-rwxS] [-s size] [off len]");
>  	mmap_cmd.oneline =
>  		_("mmap a range in the current file, show mappings");
>  	mmap_cmd.help = mmap_help;
> diff --git a/m4/package_libcdev.m4 b/m4/package_libcdev.m4
> index fdf9d69..598f5df 100644
> --- a/m4/package_libcdev.m4
> +++ b/m4/package_libcdev.m4
> @@ -314,3 +314,19 @@ AC_DEFUN([AC_HAVE_GETFSMAP],
>         AC_MSG_RESULT(no))
>      AC_SUBST(have_getfsmap)
>    ])
> +
> +#
> +# Check if we have MAP_SYNC defines (Linux)
> +#
> +AC_DEFUN([AC_HAVE_MAP_SYNC],
> +  [ AC_MSG_CHECKING([for MAP_SYNC])
> +    AC_TRY_COMPILE([
> +#include <asm-generic/mman.h>
> +#include <asm-generic/mman-common.h>
> +    ], [
> +         int flags = MAP_SHARED_VALIDATE | MAP_SYNC;
> +    ], have_map_sync=yes
> +       AC_MSG_RESULT(yes),
> +       AC_MSG_RESULT(no))
> +    AC_SUBST(have_map_sync)
> +  ])
> diff --git a/man/man8/xfs_io.8 b/man/man8/xfs_io.8
> index 9bf1a47..1693f7f 100644
> --- a/man/man8/xfs_io.8
> +++ b/man/man8/xfs_io.8
> @@ -764,7 +764,7 @@ Each (sec, nsec) pair constitutes a single timestamp value.
>  
>  .SH MEMORY MAPPED I/O COMMANDS
>  .TP
> -.BI "mmap [ " N " | [[ \-rwx ] [\-s " size " ] " "offset length " ]]
> +.BI "mmap [ " N " | [[ \-rwxS ] [\-s " size " ] " "offset length " ]]
>  With no arguments,
>  .B mmap
>  shows the current mappings. Specifying a single numeric argument
> @@ -780,6 +780,10 @@ PROT_WRITE
>  .RB ( \-w ),
>  and PROT_EXEC
>  .RB ( \-x ).
> +The mapping will be created with the MAP_SHARED flag by default, or with the
> +Linux specific (MAP_SYNC | MAP_SHARED_VALIDATE) flags if
> +.B -S
> +is given.

Woo, documentation!

--D

>  .BI \-s " size"
>  is used to do a mmap(size) && munmap(size) operation at first, try to reserve some
>  extendible free memory space, if
> -- 
> 2.9.5
> 
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Ross Zwisler Nov. 17, 2017, 9:33 p.m. UTC | #3 
On Fri, Nov 17, 2017 at 12:35:43PM -0800, Dan Williams wrote:
> On Fri, Nov 17, 2017 at 12:25 PM, Ross Zwisler
> <ross.zwisler@linux.intel.com> wrote:
> > Add support for a new -S flag to xfs_io's mmap command.  This opens the
> > mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
> > standard MAP_SHARED flag.
> >
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > Suggested-by: Dave Chinner <david@fromorbit.com>
> > ---
> >  configure.ac          |  1 +
> >  include/builddefs.in  |  1 +
> >  include/linux.h       |  5 +++++
> >  io/io.h               |  1 +
> >  io/mmap.c             | 19 ++++++++++++++-----
> >  m4/package_libcdev.m4 | 16 ++++++++++++++++
> >  man/man8/xfs_io.8     |  6 +++++-
> >  7 files changed, 43 insertions(+), 6 deletions(-)
> >
> > diff --git a/configure.ac b/configure.ac
> > index cb23fb8..7153c9f 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -162,6 +162,7 @@ AC_HAVE_FSETXATTR
> >  AC_HAVE_MREMAP
> >  AC_NEED_INTERNAL_FSXATTR
> >  AC_HAVE_GETFSMAP
> > +AC_HAVE_MAP_SYNC
> >
> >  if test "$enable_blkid" = yes; then
> >  AC_HAVE_BLKID_TOPO
> > diff --git a/include/builddefs.in b/include/builddefs.in
> > index 1d454b6..78b71fe 100644
> > --- a/include/builddefs.in
> > +++ b/include/builddefs.in
> > @@ -114,6 +114,7 @@ HAVE_FSETXATTR = @have_fsetxattr@
> >  HAVE_MREMAP = @have_mremap@
> >  NEED_INTERNAL_FSXATTR = @need_internal_fsxattr@
> >  HAVE_GETFSMAP = @have_getfsmap@
> > +HAVE_MAP_SYNC = @have_map_sync@
> >
> >  GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
> >  #         -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-decl
> > diff --git a/include/linux.h b/include/linux.h
> > index 6ce344c..4ee03ed 100644
> > --- a/include/linux.h
> > +++ b/include/linux.h
> > @@ -327,4 +327,9 @@ fsmap_advance(
> >  #define HAVE_GETFSMAP
> >  #endif /* HAVE_GETFSMAP */
> >
> > +#ifndef HAVE_MAP_SYNC
> > +#define MAP_SYNC 0x80000
> 
> Hmm, this is the x86 specific value of MAP_SYNC. I think it would be
> better to define MAP_SYNC to zero in this case and check for MAP_SYNC
> == 0 at run time.

Ah, so instead of defining MAP_SYNC if the proper headers aren't installed,
set to 0 and disallow the -S option?  Fair enough.
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Ross Zwisler Nov. 17, 2017, 9:44 p.m. UTC | #4 
On Fri, Nov 17, 2017 at 12:40:21PM -0800, Darrick J. Wong wrote:
> On Fri, Nov 17, 2017 at 01:25:23PM -0700, Ross Zwisler wrote:
> > Add support for a new -S flag to xfs_io's mmap command.  This opens the
> > mapping with the (MAP_SYNC | MAP_SHARED_VALIDATE) flags instead of the
> > standard MAP_SHARED flag.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > Suggested-by: Dave Chinner <david@fromorbit.com>
> > ---
<>
> > diff --git a/include/linux.h b/include/linux.h
> > index 6ce344c..4ee03ed 100644
> > --- a/include/linux.h
> > +++ b/include/linux.h
> > @@ -327,4 +327,9 @@ fsmap_advance(
> >  #define HAVE_GETFSMAP
> >  #endif /* HAVE_GETFSMAP */
> >  
> > +#ifndef HAVE_MAP_SYNC
> > +#define MAP_SYNC 0x80000
> > +#define MAP_SHARED_VALIDATE 0x3
> > +#endif /* HAVE_MAP_SYNC */
> 
> Hmm, what's the point of ifndef/define if you have an configure.ac check?

I'm following the example of HAVE_GETFSMAP.  It does a check to see if the
headers have proper support in m4/package_libcdev.m4, then has code in this
file to provide defines if they aren't provided in the system.

> > diff --git a/io/mmap.c b/io/mmap.c
> > index 7a8150e..520b037 100644
> > --- a/io/mmap.c
> > +++ b/io/mmap.c
> > @@ -42,7 +42,7 @@ print_mapping(
> >  	int		index,
> >  	int		braces)
> >  {
> > -	unsigned char	buffer[8] = { 0 };
> > +	char		buffer[8] = { 0 };
> >  	int		i;
> >  
> >  	static struct {
> > @@ -57,6 +57,10 @@ print_mapping(
> >  
> >  	for (i = 0, p = pflags; p->prot != PROT_NONE; i++, p++)
> >  		buffer[i] = (map->prot & p->prot) ? p->mode : '-';
> > +
> > +	if (map->map_sync)
> > +		sprintf(&buffer[i], " S");
> 
> Does buffer need enlarging here?

Nope.  The buffer is 8 chars, and the 'rwx\0' string only uses 4.
"rwx S\0" uses 6, so we're still good to go.
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diff mbox

Patch

diff --git a/configure.ac b/configure.ac
index cb23fb8..7153c9f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -162,6 +162,7 @@  AC_HAVE_FSETXATTR
 AC_HAVE_MREMAP
 AC_NEED_INTERNAL_FSXATTR
 AC_HAVE_GETFSMAP
+AC_HAVE_MAP_SYNC
 
 if test "$enable_blkid" = yes; then
 AC_HAVE_BLKID_TOPO
diff --git a/include/builddefs.in b/include/builddefs.in
index 1d454b6..78b71fe 100644
--- a/include/builddefs.in
+++ b/include/builddefs.in
@@ -114,6 +114,7 @@  HAVE_FSETXATTR = @have_fsetxattr@
 HAVE_MREMAP = @have_mremap@
 NEED_INTERNAL_FSXATTR = @need_internal_fsxattr@
 HAVE_GETFSMAP = @have_getfsmap@
+HAVE_MAP_SYNC = @have_map_sync@
 
 GCCFLAGS = -funsigned-char -fno-strict-aliasing -Wall
 #	   -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-decl
diff --git a/include/linux.h b/include/linux.h
index 6ce344c..4ee03ed 100644
--- a/include/linux.h
+++ b/include/linux.h
@@ -327,4 +327,9 @@  fsmap_advance(
 #define HAVE_GETFSMAP
 #endif /* HAVE_GETFSMAP */
 
+#ifndef HAVE_MAP_SYNC
+#define MAP_SYNC 0x80000
+#define MAP_SHARED_VALIDATE 0x3
+#endif /* HAVE_MAP_SYNC */
+
 #endif	/* __XFS_LINUX_H__ */
diff --git a/io/io.h b/io/io.h
index 3862985..8b2753b 100644
--- a/io/io.h
+++ b/io/io.h
@@ -65,6 +65,7 @@  typedef struct mmap_region {
 	size_t		length;		/* length of mapping */
 	off64_t		offset;		/* start offset into backing file */
 	int		prot;		/* protection mode of the mapping */
+	bool		map_sync;	/* is this a MAP_SYNC mapping? */
 	char		*name;		/* name of backing file */
 } mmap_region_t;
 
diff --git a/io/mmap.c b/io/mmap.c
index 7a8150e..520b037 100644
--- a/io/mmap.c
+++ b/io/mmap.c
@@ -42,7 +42,7 @@  print_mapping(
 	int		index,
 	int		braces)
 {
-	unsigned char	buffer[8] = { 0 };
+	char		buffer[8] = { 0 };
 	int		i;
 
 	static struct {
@@ -57,6 +57,10 @@  print_mapping(
 
 	for (i = 0, p = pflags; p->prot != PROT_NONE; i++, p++)
 		buffer[i] = (map->prot & p->prot) ? p->mode : '-';
+
+	if (map->map_sync)
+		sprintf(&buffer[i], " S");
+
 	printf("%c%03d%c 0x%lx - 0x%lx %s  %14s (%lld : %ld)\n",
 		braces? '[' : ' ', index, braces? ']' : ' ',
 		(unsigned long)map->addr,
@@ -146,6 +150,7 @@  mmap_help(void)
 " -r -- map with PROT_READ protection\n"
 " -w -- map with PROT_WRITE protection\n"
 " -x -- map with PROT_EXEC protection\n"
+" -S -- map with MAP_SHARED_VALIDATE and MAP_SYNC flags\n"
 " -s <size> -- first do mmap(size)/munmap(size), try to reserve some free space\n"
 " If no protection mode is specified, all are used by default.\n"
 "\n"));
@@ -161,7 +166,7 @@  mmap_f(
 	void		*address = NULL;
 	char		*filename;
 	size_t		blocksize, sectsize;
-	int		c, prot = 0;
+	int		c, prot = 0, flags = MAP_SHARED;
 
 	if (argc == 1) {
 		if (mapping)
@@ -184,7 +189,7 @@  mmap_f(
 
 	init_cvtnum(&blocksize, &sectsize);
 
-	while ((c = getopt(argc, argv, "rwxs:")) != EOF) {
+	while ((c = getopt(argc, argv, "rwxSs:")) != EOF) {
 		switch (c) {
 		case 'r':
 			prot |= PROT_READ;
@@ -195,6 +200,9 @@  mmap_f(
 		case 'x':
 			prot |= PROT_EXEC;
 			break;
+		case 'S':
+			flags = MAP_SHARED_VALIDATE | MAP_SYNC;
+			break;
 		case 's':
 			length2 = cvtnum(blocksize, sectsize, optarg);
 			break;
@@ -238,7 +246,7 @@  mmap_f(
 		               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 		munmap(address, length2);
 	}
-	address = mmap(address, length, prot, MAP_SHARED, file->fd, offset);
+	address = mmap(address, length, prot, flags, file->fd, offset);
 	if (address == MAP_FAILED) {
 		perror("mmap");
 		free(filename);
@@ -263,6 +271,7 @@  mmap_f(
 	mapping->offset = offset;
 	mapping->name = filename;
 	mapping->prot = prot;
+	mapping->map_sync = (flags == (MAP_SHARED_VALIDATE | MAP_SYNC));
 	return 0;
 }
 
@@ -676,7 +685,7 @@  mmap_init(void)
 	mmap_cmd.argmax = -1;
 	mmap_cmd.flags = CMD_NOMAP_OK | CMD_NOFILE_OK |
 			 CMD_FOREIGN_OK | CMD_FLAG_ONESHOT;
-	mmap_cmd.args = _("[N] | [-rwx] [-s size] [off len]");
+	mmap_cmd.args = _("[N] | [-rwxS] [-s size] [off len]");
 	mmap_cmd.oneline =
 		_("mmap a range in the current file, show mappings");
 	mmap_cmd.help = mmap_help;
diff --git a/m4/package_libcdev.m4 b/m4/package_libcdev.m4
index fdf9d69..598f5df 100644
--- a/m4/package_libcdev.m4
+++ b/m4/package_libcdev.m4
@@ -314,3 +314,19 @@  AC_DEFUN([AC_HAVE_GETFSMAP],
        AC_MSG_RESULT(no))
     AC_SUBST(have_getfsmap)
   ])
+
+#
+# Check if we have MAP_SYNC defines (Linux)
+#
+AC_DEFUN([AC_HAVE_MAP_SYNC],
+  [ AC_MSG_CHECKING([for MAP_SYNC])
+    AC_TRY_COMPILE([
+#include <asm-generic/mman.h>
+#include <asm-generic/mman-common.h>
+    ], [
+         int flags = MAP_SHARED_VALIDATE | MAP_SYNC;
+    ], have_map_sync=yes
+       AC_MSG_RESULT(yes),
+       AC_MSG_RESULT(no))
+    AC_SUBST(have_map_sync)
+  ])
diff --git a/man/man8/xfs_io.8 b/man/man8/xfs_io.8
index 9bf1a47..1693f7f 100644
--- a/man/man8/xfs_io.8
+++ b/man/man8/xfs_io.8
@@ -764,7 +764,7 @@  Each (sec, nsec) pair constitutes a single timestamp value.
 
 .SH MEMORY MAPPED I/O COMMANDS
 .TP
-.BI "mmap [ " N " | [[ \-rwx ] [\-s " size " ] " "offset length " ]]
+.BI "mmap [ " N " | [[ \-rwxS ] [\-s " size " ] " "offset length " ]]
 With no arguments,
 .B mmap
 shows the current mappings. Specifying a single numeric argument
@@ -780,6 +780,10 @@  PROT_WRITE
 .RB ( \-w ),
 and PROT_EXEC
 .RB ( \-x ).
+The mapping will be created with the MAP_SHARED flag by default, or with the
+Linux specific (MAP_SYNC | MAP_SHARED_VALIDATE) flags if
+.B -S
+is given.
 .BI \-s " size"
 is used to do a mmap(size) && munmap(size) operation at first, try to reserve some
 extendible free memory space, if