[systemd-devel] [PATCH 3/3] Use a stamp file to avoid running systemd-fsck-root.service twice

[Zbigniew Jędrzejewski-Szmek]

On Sun, May 03, 2015 at 06:06:58PM +0300, Andrei Borzenkov wrote:
> В Sun, 3 May 2015 16:17:15 +0200
> Lennart Poettering <lennart at poettering.net> пишет:
> 
> > On Sat, 02.05.15 13:16, Zbigniew Jędrzejewski-Szmek (zbyszek at in.waw.pl) wrote:
> > 
> > So, the last time we discussed this we figured we should do this
> > differently, and simply generate systemd-fsck-root.service in the
> > initrd as well, that uses a different command line internally. The end
> > result would then be that we can do without flag file, and always have
> > the guarantee that "systemd-fsck-root.service" is the services that
> > fsck'ed the root file system, regardless whether in initrd or not.
> > 
> 
> systemd-fsck at .service has explicit dependency on
> systemd-fsck-root.service so other mounts (/usr, anything else?) will
> be serialized after it. Currently they can run in parallel.
> 
> Not I think it is a big problem, but at least to consider.
Yeah, that's the main wart. I tried to outline it in the second bullet
point below.

I prepared a patch to generate systemd-fsck-root.service in
generator_write_fsck_deps() first, but I wasn't happy with the result.
If we ignore the dependency issue, it might be judged more elegant,
since it just uses unit stat to pass information.

> > Harald, can you comment?

Zbyszek

> > > In the initramfs, we run systemd-fsck@<sysroot-device>.service.
> > > In the real system we run systemd-fsck-root.service. It is hard
> > > to pass the information that the latter should not run if the first
> > > succeeded using unit state only.
> > > 
> > > - in the real system, we need a synchronization point between the fsck
> > >   for root and other fscks, to express the dependency to run this
> > >   systemd-fsck at .service "before all other systemd-fsck@ units". We
> > >   cannot express it directly, because there are no wildcard
> > >   dependencies. We could use a target as a sychronization point, but
> > >   then we would have to provide drop-ins to order
> > >   systemd-fsck at -.service before the target, and all others after it,
> > >   which becomes messy. The currently used alternative of having a
> > >   special unit (systemd-fsck-root.service) makes it easy to express
> > >   this dependency, and seems to be the best solution.
> > > 
> > > - we cannot use systemd-fsck-root.service in the initramfs, because
> > >   other fsck units should not be ordered after it. In the real system,
> > >   the root device is always checked and mounted before other filesystems,
> > >   but in the initramfs this doesn't have to be true: /sysroot might be
> > >   stacked on other filesystems and devices.
> > > 
> > > - the name of the root device can legitimately be specified in a
> > >   different way in the initramfs (on the kernel command line, or
> > >   automatically discovered through GPT), and in the real fs (in /etc/fstab).
> > >   Even if we didn't need systemd-fsck-root.service as a synchronization
> > >   point, it would be hard to ensure the same instance parameter is
> > >   provided for systemd-fsck at .service in the initrams and the real
> > >   system.
> > > 
> > > Let's use a "side channel" to pass this information.
> > > /run/systemd/fsck-root-done is touched after fsck in the initramfs
> > > succeeds, through an ExecStartPost line in a drop-in for
> > > systemd-fsck at sysroot.service.
> > > 
> > > https://bugzilla.redhat.com/show_bug.cgi?id=1201979
> > > ---
> > >  src/shared/generator.c             | 7 +++++++
> > >  units/systemd-fsck-root.service.in | 1 +
> > >  2 files changed, 8 insertions(+)
> > > 
> > > diff --git a/src/shared/generator.c b/src/shared/generator.c
> > > index 7b2f846175..a71222d1cb 100644
> > > --- a/src/shared/generator.c
> > > +++ b/src/shared/generator.c
> > > @@ -78,6 +78,13 @@ int generator_write_fsck_deps(
> > >                          "RequiresOverridable=%1$s\n"
> > >                          "After=%1$s\n",
> > >                          fsck);
> > > +
> > > +                if (in_initrd() && path_equal(where, "/sysroot"))
> > > +                        return write_drop_in_format(dir, fsck, 50, "stamp",
> > > +                                                    "# Automatically generated by %s\n\n"
> > > +                                                    "[Service]\n"
> > > +                                                    "ExecStartPost=-/bin/touch /run/systemd/fsck-root-done\n",
> > > +                                                    program_invocation_short_name);
> > >          }
> > >  
> > >          return 0;
> > > diff --git a/units/systemd-fsck-root.service.in b/units/systemd-fsck-root.service.in
> > > index 3617abf04a..48dacc841c 100644
> > > --- a/units/systemd-fsck-root.service.in
> > > +++ b/units/systemd-fsck-root.service.in
> > > @@ -11,6 +11,7 @@ Documentation=man:systemd-fsck-root.service(8)
> > >  DefaultDependencies=no
> > >  Before=local-fs.target shutdown.target
> > >  ConditionPathIsReadWrite=!/
> > > +ConditionPathExists=!/run/systemd/fsck-root-done
> > >  
> > >  [Service]
> > >  Type=oneshot
> > > -- 
> > > 2.3.5
> > > 
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> > 
> > 
> > Lennart
> > 
> 
>