Re: [patch] mremap.2: Add description of old_size == 0 functionality

[Mike Kravetz]

CC: linux-mm

On 09/15/2017 02:37 PM, Mike Kravetz wrote:
> Since at least the 2.6 time frame, mremap would create a new mapping
> of the same pages if 'old_size == 0'.  It would also leave the original
> mapping.  This was used to create a 'duplicate mapping'.
> 
> Document the behavior and return codes.  But, also mention that the
> functionality is deprecated and discourage its use.
> 
> A recent change was made to mremap so that an attempt to create a
> duplicate a private mapping will fail.
> 
> commit dba58d3b8c5045ad89c1c95d33d01451e3964db7
> Author: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Date:   Wed Sep 6 16:20:55 2017 -0700
> 
>     mm/mremap: fail map duplication attempts for private mappings
> 
> This return code is also documented here.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  man2/mremap.2 | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/man2/mremap.2 b/man2/mremap.2
> index 98643c640..98df7d5fa 100644
> --- a/man2/mremap.2
> +++ b/man2/mremap.2
> @@ -58,6 +58,21 @@ may be provided; see the description of
>  .B MREMAP_FIXED
>  below.
>  .PP
> +If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to
> +a private anonymous mapping, then
> +.BR mremap ()
> +will create a new mapping of the same pages. \fInew_size\fP
> +will be the size of the new mapping and the location of the new mapping
> +may be specified with \fInew_address\fP, see the description of
> +.B MREMAP_FIXED
> +below.  If a new mapping is requested via this method, then the
> +.B MREMAP_MAYMOVE
> +flag must also be specified.  This functionality is deprecated, and no
> +new code should be written to use this feature.  A better method of
> +obtaining multiple mappings of the same private anonymous memory is via the
> +.BR memfd_create()
> +system call.
> +.PP
>  In Linux the memory is divided into pages.
>  A user process has (one or)
>  several linear virtual memory segments.
> @@ -174,7 +189,12 @@ and
>  or
>  .B MREMAP_FIXED
>  was specified without also specifying
> -.BR MREMAP_MAYMOVE .
> +.BR MREMAP_MAYMOVE ;
> +or \fIold_size\fP was zero and \fIold_address\fP does not refer to a
> +private anonymous mapping;
> +or \fIold_size\fP was zero and the
> +.BR MREMAP_MAYMOVE
> +flag was not specified.
>  .TP
>  .B ENOMEM
>  The memory area cannot be expanded at the current virtual address, and the
> @@ -210,6 +230,7 @@ if the area cannot be populated.
>  .BR brk (2),
>  .BR getpagesize (2),
>  .BR getrlimit (2),
> +.BR memfd_create(2),
>  .BR mlock (2),
>  .BR mmap (2),
>  .BR sbrk (2),
>