Hi Lorenzo,
On Mon, Aug 11, 2025 at 03:59:38PM +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.
>
> 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;
> }
I've applied some editorial changes to the program.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Thanks! I've applied the patch, with a small amendment (see below).
<https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=d99a3495372a69b48309f2a1a4e2067af2bfbe69>
Have a lovely day!
Alex
> ---
> man/man2/mremap.2 | 68 +++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 57 insertions(+), 11 deletions(-)
>
> diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
> index 4e3c8e54e..6d14bf627 100644
> --- a/man/man2/mremap.2
> +++ b/man/man2/mremap.2
> @@ -35,22 +35,36 @@ and using the
> .B MREMAP_FIXED
> flag
> (see below).
> +Since Linux 6.17,
> +while
> +.I old_address
> +must be mapped,
> +.I old_size
> +may span multiple mappings
> +including unmapped areas between
> +them when performing a move like this.
And I've reworded this last line. Repetitive consistent language helps
understanding documentation, so it's better to repeat "a simple move"
here again.
diff --git i/man/man2/mremap.2 w/man/man2/mremap.2
index 6d14bf627..65b4d5f58 100644
--- i/man/man2/mremap.2
+++ w/man/man2/mremap.2
@@ -42,7 +42,7 @@ .SH DESCRIPTION
.I old_size
may span multiple mappings
including unmapped areas between
-them when performing a move like this.
+them when performing a simple move.
The
.B MREMAP_DONTUNMAP
flag may also be specified.
> The
> .B MREMAP_DONTUNMAP
> flag may also 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
> @@ -119,13 +133,42 @@ 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
> +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
> @@ -163,13 +206,13 @@ 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
> @@ -202,7 +245,10 @@ 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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