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Subject[patch v2] mremap.2: Add description of old_size == 0 functionality
v2: Fix incorrect wording noticed by Jann Horn.
Remove deprecated and memfd_create discussion as suggested
by Florian Weimer.

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'.

A recent change was made to mremap so that an attempt to create a
duplicate a private mapping will fail.

Document the 'old_size == 0' behavior and new return code from
below commit.

commit dba58d3b8c5045ad89c1c95d33d01451e3964db7
Author: Mike Kravetz <>
Date: Wed Sep 6 16:20:55 2017 -0700

mm/mremap: fail map duplication attempts for private mappings

Signed-off-by: Mike Kravetz <>
man2/mremap.2 | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/man2/mremap.2 b/man2/mremap.2
index 98643c640..235984a96 100644
--- a/man2/mremap.2
+++ b/man2/mremap.2
@@ -58,6 +58,20 @@ may be provided; see the description of
+If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to
+a shareable mapping (see
+.BR mmap (2)
+, 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
+below. If a new mapping is requested via this method, then the
+flag must also be specified.
In Linux the memory is divided into pages.
A user process has (one or)
several linear virtual memory segments.
@@ -174,7 +188,12 @@ and
was specified without also specifying
+or \fIold_size\fP was zero and \fIold_address\fP does not refer to a
+shareable mapping;
+or \fIold_size\fP was zero and the
+flag was not specified.
The memory area cannot be expanded at the current virtual address, and the
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