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Subject[patch] mremap.2: Add description of old_size == 0 functionality
Date
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@oracle.com>
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@oracle.com>
---
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),
--
2.13.5
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