[PATCH] man/man2/mremap.2: describe multiple mapping move, shrink

[PATCH] man/man2/mremap.2: describe multiple mapping move, shrink

[Lorenzo Stoakes]

There is pre-existing logic that appears to be undocumented for an mremap()
shrink operation, where it turns out that the usual 'input range must span
a single mapping' requirement no longer applies.

In fact, it turns out that the input range specified by [old_address,
old_size) may span any number of mappings, as long old_address resides at
or within a mapping and [old_address, new_size) spans only a single
mapping.

Explicitly document this.

In addition, 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 | 93 +++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 82 insertions(+), 11 deletions(-)

diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
index 2168ca728..c1a9e7397 100644
--- a/man/man2/mremap.2
+++ b/man/man2/mremap.2
@@ -25,18 +25,56 @@ 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
+.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
-is the old address of the virtual memory block that you
-want to expand (or shrink).
+must reside within a mapping,
+.I old_size
+may span multiple mappings
+which do not have to be
+adjacent to one another.
+.P
+Equally, if the operation performs a shrink,
+that is if
+.I old_size
+is greater than
+.IR new_size ,
+then
+.I old_size
+may also span multiple mappings
+which do not have to be
+adjacent to one another.
+However in this case,
+.I new_size
+must span only a single mapping.
+.P
+If the operation is neither a simple move
+nor a shrink,
+then
+.I old_size
+must span only a single mapping.
+.P
+.I old_address
+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 +143,43 @@ 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
+
+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 +217,13 @@ 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 +256,10 @@ 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

Re: [PATCH] man/man2/mremap.2: describe multiple mapping move, shrink

[Alejandro Colomar]

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Hi Lorenzo,

On Wed, Jul 23, 2025 at 06:46:34PM +0100, Lorenzo Stoakes wrote:
> There is pre-existing logic that appears to be undocumented for an mremap()
> shrink operation, where it turns out that the usual 'input range must span
> a single mapping' requirement no longer applies.
> 
> In fact, it turns out that the input range specified by [old_address,
> old_size) may span any number of mappings, as long old_address resides at
> or within a mapping and [old_address, new_size) spans only a single
> mapping.
> 
> Explicitly document this.
> 
> In addition, 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.

Please separate the new behavior into a separate patch.  Each patch
should change one thing only.


Have a lovely night!
Alex

> 
> 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 | 93 +++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 82 insertions(+), 11 deletions(-)
> 
> diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
> index 2168ca728..c1a9e7397 100644
> --- a/man/man2/mremap.2
> +++ b/man/man2/mremap.2
> @@ -25,18 +25,56 @@ 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
> +.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
> -is the old address of the virtual memory block that you
> -want to expand (or shrink).
> +must reside within a mapping,
> +.I old_size
> +may span multiple mappings
> +which do not have to be
> +adjacent to one another.
> +.P
> +Equally, if the operation performs a shrink,
> +that is if
> +.I old_size
> +is greater than
> +.IR new_size ,
> +then
> +.I old_size
> +may also span multiple mappings
> +which do not have to be
> +adjacent to one another.
> +However in this case,
> +.I new_size
> +must span only a single mapping.
> +.P
> +If the operation is neither a simple move
> +nor a shrink,
> +then
> +.I old_size
> +must span only a single mapping.
> +.P
> +.I old_address
> +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 +143,43 @@ 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
> +
> +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 +217,13 @@ 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 +256,10 @@ 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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Re: [PATCH] man/man2/mremap.2: describe multiple mapping move, shrink

[Lorenzo Stoakes]

On Fri, Jul 25, 2025 at 10:44:59PM +0200, Alejandro Colomar wrote:
> Hi Lorenzo,
>
> On Wed, Jul 23, 2025 at 06:46:34PM +0100, Lorenzo Stoakes wrote:
> > There is pre-existing logic that appears to be undocumented for an mremap()
> > shrink operation, where it turns out that the usual 'input range must span
> > a single mapping' requirement no longer applies.
> >
> > In fact, it turns out that the input range specified by [old_address,
> > old_size) may span any number of mappings, as long old_address resides at
> > or within a mapping and [old_address, new_size) spans only a single
> > mapping.
> >
> > Explicitly document this.
> >
> > In addition, 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.
>
> Please separate the new behavior into a separate patch.  Each patch
> should change one thing only.

OK will split and send two separate patches. Since this will cause merge pain
otherwise, I'll send it as a series.