Re: [patch] mremap.2: Add description of old_size == 0 functionality
From: Mike Kravetz
Date: Fri Sep 15 2017 - 17:55:14 EST
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
> +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.
> In Linux the memory is divided into pages.
> A user process has (one or)
> several linear virtual memory segments.
> @@ -174,7 +189,12 @@ and
> .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.
> .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),