[PATCH] io/mmap: new -s option for mmap command to reserve some free space

Brian Foster bfoster at redhat.com
Wed Mar 16 09:22:53 CDT 2016 

On Wed, Mar 16, 2016 at 09:53:43PM +0800, Zorro Lang wrote:
> On Wed, Mar 16, 2016 at 08:31:14AM -0400, Brian Foster wrote:
> > On Wed, Mar 16, 2016 at 05:18:04PM +0800, Zorro Lang wrote:
> > > This patch come from a test likes below:
> > > xfs_io -t -f
> > >        -c "truncate 10000"
> > >        -c "mmap -rw 0 1024"
> > >        -c "mremap 8192"
> > >        file
> > > 
> > > mremap always hit ENOMEM error, when it try to remap more space
> > > without MREMAP_MAYMOVE flag. This's a normal condition, due to
> > > no free space after mapped 1024 bytes region.
> > > 
> > > But if we try to mremap from the original mapped starting point in
> > > a C program, at first we always do:
> > > 
> > >   addr = mmap(NULL, res_size, prot, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > >   munmap(addr, res_size);
> > > 
> > > Then do:
> > > 
> > >   addr = mmap(addr, real_len, ...);
> > > 
> > > The "res_size" is bigger than "real_len". This will help us get a
> > > region between real_len and res_size, which maybe free space. The
> > > truth is we can't guarantee that this free memory will stay free.
> > > But this method is still very helpful for reserve some free space
> > > in short time.
> > > 
> > > After merge this patch, we can resolve above mremap problem by run:
> > > xfs_io -t -f
> > >        ...
> > >        -c "mmap -rw -s 8192 0 1024"
> > >        -c "mremap 8192"
> > >        ...
> > > 
> > > Although we can't sure it's useful 100%, it really have pretty high
> > > success rate.
> > > 
> > > Signed-off-by: Zorro Lang <zlang at redhat.com>
> > > ---
> > 
> > I'm a little curious why one would use this as opposed to 'mremap -m' in
> > the context of xfs_io (it certainly might make sense for an
> > application). It sounds like any xfstests tests, for example, that is
> > susceptible to this problem might want to use -m even if -s is employed
> > as well. Can you provide any additional context on this or do you have a
> > use case in mind?
> > 
> > That said, I'm not against adding this to the xfs_io toolbox and the
> > code looks Ok to me:
> 
> Hi Brian,
> 
> I hit this problem when I try to use xfs_io to rewrite LTP's mmap16
> testcase for xfstests(I know I can backport that mmap16.c into
> xfstests/src directly, but I just try to use xfs_io). That case
> from linux commit:
> 
>   90a8020 vfs: fix data corruption when blocksize < pagesize for
>   mmaped data
> 
> For reproduce that bug, I think I shouldn't move page when mremap. So
> I need mremap to extend mapped space from orignal starting address.
> 
> If no this option, xfs_io -c "mmap 0 1024" -c "mremap 8192" will
> hit ENOMEM error nearly 100%, but if use this option, it nearly always
> remap successfully(on my test machine). So I think it maybe helpful
> in the future, if someone try to mremap without "-m" :)
> 

Sounds good, thanks for the explanation! I might suggest to include this
use case in the commit log description for future reference (perhaps
condense it a bit first).

Brian

> Thanks,
> Zorro
> 
> > 
> > Reviewed-by: Brian Foster <bfoster at redhat.com>
> > 
> > >  io/mmap.c         | 29 +++++++++++++++++++++++------
> > >  man/man8/xfs_io.8 | 17 ++++++++++++++++-
> > >  2 files changed, 39 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/io/mmap.c b/io/mmap.c
> > > index 5970069..6cd37a9 100644
> > > --- a/io/mmap.c
> > > +++ b/io/mmap.c
> > > @@ -146,6 +146,7 @@ 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 <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"));
> > >  }
> > > @@ -156,8 +157,8 @@ mmap_f(
> > >  	char		**argv)
> > >  {
> > >  	off64_t		offset;
> > > -	ssize_t		length;
> > > -	void		*address;
> > > +	ssize_t		length = 0, length2 = 0;
> > > +	void		*address = NULL;
> > >  	char		*filename;
> > >  	size_t		blocksize, sectsize;
> > >  	int		c, prot = 0;
> > > @@ -181,7 +182,9 @@ mmap_f(
> > >  		return 0;
> > >  	}
> > >  
> > > -	while ((c = getopt(argc, argv, "rwx")) != EOF) {
> > > +	init_cvtnum(&blocksize, &sectsize);
> > > +
> > > +	while ((c = getopt(argc, argv, "rwxs:")) != EOF) {
> > >  		switch (c) {
> > >  		case 'r':
> > >  			prot |= PROT_READ;
> > > @@ -192,6 +195,9 @@ mmap_f(
> > >  		case 'x':
> > >  			prot |= PROT_EXEC;
> > >  			break;
> > > +		case 's':
> > > +			length2 = cvtnum(blocksize, sectsize, optarg);
> > > +			break;
> > >  		default:
> > >  			return command_usage(&mmap_cmd);
> > >  		}
> > > @@ -202,7 +208,6 @@ mmap_f(
> > >  	if (optind != argc - 2)
> > >  		return command_usage(&mmap_cmd);
> > >  
> > > -	init_cvtnum(&blocksize, &sectsize);
> > >  	offset = cvtnum(blocksize, sectsize, argv[optind]);
> > >  	if (offset < 0) {
> > >  		printf(_("non-numeric offset argument -- %s\n"), argv[optind]);
> > > @@ -221,7 +226,19 @@ mmap_f(
> > >  		return 0;
> > >  	}
> > >  
> > > -	address = mmap(NULL, length, prot, MAP_SHARED, file->fd, offset);
> > > +	/*
> > > +	 * mmap and munmap memory area of length2 region is helpful to
> > > +	 * make a region of extendible free memory. It's generally used
> > > +	 * for later mremap operation(no MREMAP_MAYMOVE flag). But there
> > > +	 * isn't guarantee that the memory after length (up to length2)
> > > +	 * will stay free.
> > > +	 */
> > > +	if (length2 > length) {
> > > +		address = mmap(NULL, length2, prot,
> > > +		               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > > +		munmap(address, length2);
> > > +	}
> > > +	address = mmap(address, length, prot, MAP_SHARED, file->fd, offset);
> > >  	if (address == MAP_FAILED) {
> > >  		perror("mmap");
> > >  		free(filename);
> > > @@ -647,7 +664,7 @@ mmap_init(void)
> > >  	mmap_cmd.argmin = 0;
> > >  	mmap_cmd.argmax = -1;
> > >  	mmap_cmd.flags = CMD_NOMAP_OK | CMD_NOFILE_OK | CMD_FOREIGN_OK;
> > > -	mmap_cmd.args = _("[N] | [-rwx] [off len]");
> > > +	mmap_cmd.args = _("[N] | [-rwx] [-s size] [off len]");
> > >  	mmap_cmd.oneline =
> > >  		_("mmap a range in the current file, show mappings");
> > >  	mmap_cmd.help = mmap_help;
> > > diff --git a/man/man8/xfs_io.8 b/man/man8/xfs_io.8
> > > index 33fbe6a..93a8a00 100644
> > > --- a/man/man8/xfs_io.8
> > > +++ b/man/man8/xfs_io.8
> > > @@ -559,7 +559,7 @@ Do not print timing statistics at all.
> > >  
> > >  .SH MEMORY MAPPED I/O COMMANDS
> > >  .TP
> > > -.BI "mmap [ " N " | [[ \-rwx ] " "offset length " ]]
> > > +.BI "mmap [ " N " | [[ \-rwx ] [\-s " size " ] " "offset length " ]]
> > >  With no arguments,
> > >  .B mmap
> > >  shows the current mappings. Specifying a single numeric argument
> > > @@ -575,6 +575,21 @@ PROT_WRITE
> > >  .RB ( \-w ),
> > >  and PROT_EXEC
> > >  .RB ( \-x ).
> > > +.BI \-s " size"
> > > +is used to do a mmap(size) && munmap(size) operation at first, try to reserve some
> > > +extendible free memory space, if
> > > +.I size
> > > +is bigger than
> > > +.I length
> > > +parameter. But there's not guarantee that the memory after
> > > +.I length
> > > +( up to
> > > +.I size
> > > +) will stay free.
> > > +.B e.g.
> > > +"mmap -rw -s 8192 1024" will mmap 0 ~ 1024 bytes memory, but try to reserve 1024 ~ 8192
> > > +free space(no guarantee). This free space will helpful for "mremap 8192" without
> > > +MREMAP_MAYMOVE flag.
> > >  .TP
> > >  .B mm
> > >  See the
> > 
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> 
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