Re: [PATCH v4 1/2] man/man2/mremap.2: describe multiple mapping move

[Alejandro Colomar <alx@kernel.org>]

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Hi Lorenzo,

On Tue, Aug 05, 2025 at 06:31:55PM +0100, Lorenzo Stoakes 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.

Can we split this further into two commits?

> 
> 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.
> 
> Example code is enclosed below demonstrating the behaviour which is now
> possible:
> 
> int main(void)
> {
> 	unsigned long page_size = sysconf(_SC_PAGESIZE);
> 	void *ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE,
> 			 MAP_ANON | MAP_PRIVATE, -1, 0);
> 	void *tgt_ptr = mmap(NULL, 10 * page_size, PROT_NONE,
> 			     MAP_ANON | MAP_PRIVATE, -1, 0);
> 	int i;
> 
> 	if (ptr == MAP_FAILED || tgt_ptr == MAP_FAILED) {
> 		perror("mmap");
> 		return EXIT_FAILURE;
> 	}
> 
> 	/* Unmap every other page. */
> 	for (i = 1; i < 10; i += 2)
> 		munmap(ptr + i * page_size, page_size);
> 
> 	/* Now move all 5 distinct mappings to tgt_ptr. */
> 	ptr = mremap(ptr, 10 * page_size, 10 * page_size,
> 		     MREMAP_MAYMOVE | MREMAP_FIXED, tgt_ptr);
> 	if (ptr == MAP_FAILED) {
> 		perror("mremap");
> 		return EXIT_FAILURE;
> 	}
> 
> 	return EXIT_SUCCESS;
> }
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  man/man2/mremap.2 | 84 ++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 73 insertions(+), 11 deletions(-)
> 
> diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
> index 2168ca728..6ba51310c 100644
> --- a/man/man2/mremap.2
> +++ b/man/man2/mremap.2
> @@ -25,18 +25,47 @@ moving it at the same time (controlled by the
>  argument and
>  the available virtual address space).
>  .P
> +Mappings can also simply be moved
> +(without any resizing)
> +by specifying equal
> +.I old_size
> +and
> +.I new_size
> +and using the
> +.B MREMAP_FIXED
> +flag
> +(see below).
> +Since Linux 6.17,
> +while
> +.I old_address
> +must reside within a mapping,

I don't understand this.  What does it mean that old_address must reside
within a mapping?  It's a point, not a size, so I'm not sure I
understand it.

> +.I old_size
> +may span multiple mappings
> +which do not have to be
> +adjacent to one another when
> +performing a move like this.
> +The
> +.B MREMAP_DONTUNMAP
> +flag may be specified.
> +.P
> +If the operation is not
> +simply moving mappings,
> +then
> +.I old_size
> +must span only a single mapping.
> +.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 +134,43 @@ 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.
> +.IP
> +

Why this blank?


Cheers,
Alex

> +Moving multiple mappings is not permitted if
> +any of those mappings have either
> +been registered with
> +.BR userfaultfd (2) ,
> +or map drivers that
> +specify their own custom address mapping logic.
>  .TP
>  .BR MREMAP_DONTUNMAP " (since Linux 5.7)"
>  .\" commit e346b3813067d4b17383f975f197a9aa28a3b077
>  This flag, which must be used in conjunction with
>  .BR MREMAP_MAYMOVE ,
> -remaps a mapping to a new address but does not unmap the mapping at
> -.IR old_address .
> +remaps mappings to a new address but does not unmap them
> +from their original address.
>  .IP
>  The
>  .B MREMAP_DONTUNMAP
> @@ -149,13 +208,13 @@ mapped.
>  See NOTES for some possible applications of
>  .BR MREMAP_DONTUNMAP .
>  .P
> -If the memory segment specified by
> +If the memory segments specified by
>  .I old_address
>  and
>  .I old_size
> -is locked (using
> +are locked (using
>  .BR mlock (2)
> -or similar), then this lock is maintained when the segment is
> +or similar), then this lock is maintained when the segments are
>  resized and/or relocated.
>  As a consequence, the amount of memory locked by the process may change.
>  .SH RETURN VALUE
> @@ -188,7 +247,10 @@ virtual memory address for this process.
>  You can also get
>  .B EFAULT
>  even if there exist mappings that cover the
> -whole address space requested, but those mappings are of different types.
> +whole address space requested, but those mappings are of different types,
> +and the
> +.BR mremap ()
> +operation being performed does not support this.
>  .TP
>  .B EINVAL
>  An invalid argument was given.
> -- 
> 2.50.1
> 

-- 
<https://www.alejandro-colomar.es/>

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