From aa002e8145e643dd7f3a6a1cbe20d5291738438a Mon Sep 17 00:00:00 2001 From: Mike Kravetz Date: Tue, 19 Sep 2017 14:42:24 -0700 Subject: [PATCH] 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 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 Reviewed-by: Jann Horn Signed-off-by: Mike Kravetz Signed-off-by: Michael Kerrisk --- 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