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Thu, 21 May 2026 10:57:48 -0700 (PDT) Date: Thu, 21 May 2026 17:57:47 +0000 From: Pasha Tatashin To: luca.boccassi@gmail.com Cc: kexec@lists.infradead.org, linux-mm@kvack.org, pasha.tatashin@soleen.com, pratyush@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] liveupdate: document systemd support Message-ID: References: <20260519152957.548119-1-luca.boccassi@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260519152957.548119-1-luca.boccassi@gmail.com> X-Stat-Signature: nito71fwo6toeza18ofkxd6rh6iq8fe8 X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 69D73180004 X-HE-Tag: 1779386270-146961 X-HE-Meta: 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 wu6T5brn 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 05-19 16:28, luca.boccassi@gmail.com wrote: > From: Luca Boccassi > > 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 > --- > 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 `_ > +- `systemd.service(5) FileDescriptorStorePreserve= > + `_ > +- `sd_pid_notify_with_fds(3) > + `_ > + Reviewed-by: Pasha Tatashin > See Also > ======== > > -- > 2.47.3 >