Re: [PATCH v2 1/2] man/man2/mremap.2: describe multiple mapping move

[Jann Horn <jannh@google.com>]

On Mon, Jul 28, 2025 at 4:05 PM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
> Document the new behaviour introduced in Linux 6.17 whereby it is now
> possible to move multiple mappings in a single operation, as long as the
> operation is purely a move, that is old_size is equal to new_size and
> MREMAP_FIXED is specified.
>
> To make things clearer, also describe this 'pure move' operation, before
> expanding upon it to describe the newly introduced behaviour.
>
> This change also explains the limitations of of this method and the
> possibility of partial failure.
>
> Finally, we pluralise language where it makes sense to so the documentation
> does not contradict either this new capability nor the pre-existing edge
> case.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  man/man2/mremap.2 | 78 ++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 67 insertions(+), 11 deletions(-)
>
> diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
> index 2168ca728..cb3412591 100644
> --- a/man/man2/mremap.2
> +++ b/man/man2/mremap.2
> @@ -25,18 +25,41 @@ moving it at the same time (controlled by the
>  argument and
>  the available virtual address space).
>  .P
> +Mappings can simply be moved by specifying equal

(Bikeshedding: This "simply" sounds weird to me. If you're trying to
define a "simple move" with this, the rest of this block is not very
specific about what exactly that is supposed to be. In my opinion,
"pure" would also be a nicer word than "simple" if you're looking for
an expression that means "a move that doesn't do other things".)

> +.I old_size
> +and
> +.I new_size
> +and specifying
> +.IR new_address ,
> +see the description of
> +.B MREMAP_FIXED
> +below.
> +Since Linux 6.17,
> +while
> +.I old_address
> +must reside within a mapping,
> +.I old_size
> +may span multiple mappings
> +which do not have to be
> +adjacent to one another.
> +.P
> +If the operation is not a simple move
> +then
> +.I old_size
> +must span only a single mapping.

I'm reading between the lines that "simple move" is supposed to mean
"the size is not changing and MREMAP_DONTUNMAP is not set", which then
implies that in order to actually make anything happen, MREMAP_FIXED
must be specified?

> +.P
>  .I old_address
> -is the old address of the virtual memory block that you
> -want to expand (or shrink).
> +is the old address of the first virtual memory block that you
> +want to expand, shrink, and/or move.
>  Note that
>  .I old_address
>  has to be page aligned.
>  .I old_size
> -is the old size of the
> -virtual memory block.
> +is the size of the range containing
> +virtual memory blocks to be manipulated.
>  .I new_size
>  is the requested size of the
> -virtual memory block after the resize.
> +virtual memory blocks after the resize.
>  An optional fifth argument,
>  .IR new_address ,
>  may be provided; see the description of
> @@ -105,13 +128,43 @@ If
>  is specified, then
>  .B MREMAP_MAYMOVE
>  must also be specified.
> +.IP
> +Since Linux 6.17,
> +if
> +.I old_size
> +is equal to
> +.I new_size
> +and
> +.B MREMAP_FIXED
> +is specified, then
> +.I old_size
> +may span beyond the mapping in which
> +.I old_address
> +resides.
> +In this case,
> +gaps between mappings in the original range
> +are maintained in the new range.
> +The whole operation is performed atomically
> +unless an error arises,
> +in which case the operation may be partially
> +completed,
> +that is,
> +some mappings may be moved and others not.

This is much clearer to me.