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mremap.2: Add description of old_size == 0 functionality 

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

v2: Fix incorrect wording noticed by Jann Horn.
    Remove deprecated and memfd_create() discussion as
    suggested by Florian Weimer.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Mike Kravetz 2017-09-19 14:42:24 -07:00 committed by Michael Kerrisk
parent 2c63b13e3d
commit aa002e8145
1 changed files with 20 additions and 1 deletions
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21
man2/mremap.2
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@ -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