[patch v2] mremap.2: Add description of old_size == 0 functionality

[Mike Kravetz <mike.kravetz@oracle.com>]

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 <mike.kravetz@oracle.com>
Date:   Wed Sep 6 16:20:55 2017 -0700

    mm/mremap: fail map duplication attempts for private mappings

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 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
 .B MREMAP_FIXED
 below.
 .PP
+If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to
+a shareable mapping (see
+.BR mmap (2)
+.BR MAP_SHARED )
+, 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.
+.PP
 In Linux the memory is divided into pages.
 A user process has (one or)
 several linear virtual memory segments.
@@ -174,7 +188,12 @@ 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
+shareable 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
-- 
2.13.5

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