From: Luca Boccassi <luca.boccassi@gmail.com>

systemd v261 will add native support for LUO via the file descriptor
store mechanism. Add a brief paragraph in the LUO userspace documentation
page to inform users.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
---
NOTE: the external links will mention LUO after the next release is
      published, which should happen within ~4 weeks

 Documentation/userspace-api/liveupdate.rst | 23 ++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/Documentation/userspace-api/liveupdate.rst b/Documentation/userspace-api/liveupdate.rst
index 41c0473e4f16..f6efc32fb40b 100644
--- a/Documentation/userspace-api/liveupdate.rst
+++ b/Documentation/userspace-api/liveupdate.rst
@@ -14,6 +14,29 @@ ioctl uAPI
 ===========
 .. kernel-doc:: include/uapi/linux/liveupdate.h
 
+Userspace Integration
+=====================
+
+systemd (since version v261) uses LUO to preserve its per-service file
+descriptor store across a kexec-based live update. Services opt in by setting
+``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit,
+and push file descriptors with a name into the store via
+``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``.
+
+Services may also create their own LUO sessions (via ``/dev/liveupdate``) and
+push the resulting session fds into their file descriptor store like any other
+fd. systemd detects such session fds and handles them accordingly, and
+hands the re-retrieved session fd back to the service after kexec, using the
+existing file descriptor store service interface.
+
+For details, see:
+
+- `File Descriptor Store <https://systemd.io/FILE_DESCRIPTOR_STORE/>`_
+- `systemd.service(5) FileDescriptorStorePreserve=
+  <https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#FileDescriptorStorePreserve=>`_
+- `sd_pid_notify_with_fds(3)
+  <https://www.freedesktop.org/software/systemd/man/latest/sd_pid_notify_with_fds.html>`_
+
 See Also
 ========
 
-- 
2.47.3

On Tue, 19 May 2026 16:28:39 +0100, luca.boccassi@gmail.com wrote:
> systemd v261 will add native support for LUO via the file descriptor
> store mechanism. Add a brief paragraph in the LUO userspace documentation
> page to inform users.
> 
> 

Applied, thanks!

[1/1] liveupdate: document systemd support
      commit: 1b7ce86f23a430ac91a2ed6cd88046e16751b285

Best regards,
-- 
Pasha Tatashin <pasha.tatashin@soleen.com>

On 05-19 16:28, luca.boccassi@gmail.com wrote:
> From: Luca Boccassi <luca.boccassi@gmail.com>
> 
> systemd v261 will add native support for LUO via the file descriptor
> store mechanism. Add a brief paragraph in the LUO userspace documentation
> page to inform users.
> 
> Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
> ---
> NOTE: the external links will mention LUO after the next release is
>       published, which should happen within ~4 weeks
> 
>  Documentation/userspace-api/liveupdate.rst | 23 ++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/Documentation/userspace-api/liveupdate.rst b/Documentation/userspace-api/liveupdate.rst
> index 41c0473e4f16..f6efc32fb40b 100644
> --- a/Documentation/userspace-api/liveupdate.rst
> +++ b/Documentation/userspace-api/liveupdate.rst
> @@ -14,6 +14,29 @@ ioctl uAPI
>  ===========
>  .. kernel-doc:: include/uapi/linux/liveupdate.h
>  
> +Userspace Integration
> +=====================
> +
> +systemd (since version v261) uses LUO to preserve its per-service file
> +descriptor store across a kexec-based live update. Services opt in by setting
> +``FileDescriptorStoreMax=`` and ``FileDescriptorStorePreserve=`` in their unit,
> +and push file descriptors with a name into the store via
> +``sd_pid_notify_with_fds(... "FDSTORE=1\nFDNAME=foo")``.
> +
> +Services may also create their own LUO sessions (via ``/dev/liveupdate``) and
> +push the resulting session fds into their file descriptor store like any other
> +fd. systemd detects such session fds and handles them accordingly, and
> +hands the re-retrieved session fd back to the service after kexec, using the
> +existing file descriptor store service interface.
> +
> +For details, see:
> +
> +- `File Descriptor Store <https://systemd.io/FILE_DESCRIPTOR_STORE/>`_
> +- `systemd.service(5) FileDescriptorStorePreserve=
> +  <https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#FileDescriptorStorePreserve=>`_
> +- `sd_pid_notify_with_fds(3)
> +  <https://www.freedesktop.org/software/systemd/man/latest/sd_pid_notify_with_fds.html>`_
> +

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com> 

>  See Also
>  ========
>  
> -- 
> 2.47.3
>

On Tue, May 19 2026, luca.boccassi@gmail.com wrote:

> From: Luca Boccassi <luca.boccassi@gmail.com>
>
> systemd v261 will add native support for LUO via the file descriptor
> store mechanism. Add a brief paragraph in the LUO userspace documentation
> page to inform users.
>
> Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>

Acked-by: Pratyush Yadav <pratyush@kernel.org>

But of course, we should only apply this once systemd v261 is released.

> ---
> NOTE: the external links will mention LUO after the next release is
>       published, which should happen within ~4 weeks
>

-- 
Regards,
Pratyush Yadav

On 05-21 17:25, Pratyush Yadav wrote:
> On Tue, May 19 2026, luca.boccassi@gmail.com wrote:
> 
> > From: Luca Boccassi <luca.boccassi@gmail.com>
> >
> > systemd v261 will add native support for LUO via the file descriptor
> > store mechanism. Add a brief paragraph in the LUO userspace documentation
> > page to inform users.
> >
> > Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
> 
> Acked-by: Pratyush Yadav <pratyush@kernel.org>
> 
> But of course, we should only apply this once systemd v261 is released.

I think, time wise it is fair just to add it to liveupdate/next branch 
instead of micro-mange the exact time when this lands.

Pasha

> 
> > ---
> > NOTE: the external links will mention LUO after the next release is
> >       published, which should happen within ~4 weeks
> >
> 
> -- 
> Regards,
> Pratyush Yadav