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

Cheers,

Michael


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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