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Add a brief paragraph in the LUO userspace documentation page to inform users. Signed-off-by: Luca Boccassi Acked-by: Pratyush Yadav Reviewed-by: Pasha Tatashin =20 --- 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/use= rspace-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 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D .. kernel-doc:: include/uapi/linux/liveupdate.h =20 +Userspace Integration +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +systemd (since version v261) uses LUO to preserve its per-service file +descriptor store across a kexec-based live update. Services opt in by sett= ing +``FileDescriptorStoreMax=3D`` and ``FileDescriptorStorePreserve=3D`` in th= eir unit, +and push file descriptors with a name into the store via +``sd_pid_notify_with_fds(... "FDSTORE=3D1\nFDNAME=3Dfoo")``. + +Services may also create their own LUO sessions (via ``/dev/liveupdate``) = and +push the resulting session fds into their file descriptor store like any o= ther +fd. systemd detects such session fds and handles them accordingly, and +hands the re-retrieved session fd back to the service after kexec, using t= he +existing file descriptor store service interface. + +For details, see: + +- `File Descriptor Store `_ +- `systemd.service(5) FileDescriptorStorePreserve=3D + `_ +- `sd_pid_notify_with_fds(3) + `_ + See Also =3D=3D=3D=3D=3D=3D=3D=3D =20 --=20 2.47.3