On Sat, Aug 09, 2025 at 04:25:18PM +0200, Alejandro Colomar wrote:
> 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?
OK can do.
>
> >
> > 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@xxxxxxxxxx>
> > ---
> > 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 think if it were a size it would be more confusing no?
It's an address, the address must be located within an existing memory mapping.
Will replace with 'located' for clarity.
>
> > +.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?
Oops that was a mistake, will fix on respin.
>
>
> 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/>
