lkml.org 
[lkml]   [2017]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [patch v2] mremap.2: Add description of old_size == 0 functionality
From
Date
On 09/20/2017 12:25 AM, Michael Kerrisk (man-pages) wrote:
> Hello Mike,
>
> On 09/19/2017 11:42 PM, Mike Kravetz wrote:
>> 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
>
> I've applied this, and added Reviewed-by tags for Florian and Jann.
> But, I think it's also worth noting the older, now disallowed, behavior,
> and why the behavior was changed. So I added a note in BUGS:
>
> BUGS
> Before Linux 4.14, if old_size was zero and the mapping referred
> to by old_address was a private mapping (mmap(2) MAP_PRIVATE),
> mremap() created a new private mapping unrelated to the original
> mapping. This behavior was unintended and probably unexpected in
> user-space applications (since the intention of mremap() is to
> create a new mapping based on the original mapping). Since Linux
> 4.14, mremap() fails with the error EINVAL in this scenario.
>
> Does that seem okay?

Sorry for the late reply Michael, I've been away for a few days.

Yes, the above seems okay. Thanks for your help with this.

--
Mike Kravetz

\
 
 \ /
  Last update: 2017-09-25 18:36    [W:0.193 / U:0.044 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site