[systemd-devel] [PATCH] fstab-generator: introduce rd.weak_sysroot to bypass failures in sysroot.mount

[Colin Guthrie]

'Twas brillig, and Harald Hoyer at 31/07/13 11:19 did gyre and gimble:
> On 07/30/2013 09:14 PM, Vivek Goyal wrote:
>> On Wed, Jul 31, 2013 at 12:46:22AM +0800, WANG Chao wrote:
>>> On 07/31/13 at 12:32am, WANG Chao wrote:
>>>> On 07/30/13 at 03:46pm, Zbigniew Jędrzejewski-Szmek wrote:
>>>>> On Tue, Jul 30, 2013 at 09:43:16AM -0400, Vivek Goyal wrote:
>>>>>> [CC harald]
>>>>>>
>>>>>> Not sure if this is right way to do or not but I will give more
>>>>>> background about the issue.
>>>>>>
>>>>>> This assumption seems to be built into initramfs and systemd that root
>>>>>> should always be mountable. If one can't mount root, it is a fatal
>>>>>> failure.
>>>>>>
>>>>>> But in case of kdump initramfs, this assumption is no more valid. Core
>>>>>> might be being saved to a target which is not root (say over ssh). And
>>>>>> even if mounting root fails, it is ok.
>>>>>>
>>>>>> So we kind of need a mode (possibly driven by command line option) where
>>>>>> if mouting root failed, it is ok and continue with mouting other targets
>>>>>> and kdump module will then handle errors.
>>>>> Maybe rootfsflags=nofail could do be used as this flag?
>>>>
>>>> rootflags=nofail works. Thanks.
>>>>
>>>> Although it results in a little difference between my approach, I prefer
>>>> use this one than adding another cmdline param.
>>>
>>> I just find nofail option only works when mnt device doesn't exists.
>>>
>>> What if the filesytem is corrupted? sysroot.mount will and
>>> initrd-root-fs.target will never reach.
>>
>> Right.
>>
>> In kdump environment, for most of the users default of dropping into a
>> shell does not make sense. If some server crashes some where and we are
>> not able to take dump due to some failure, most of the users will like that
>> system reboots automatically and services are back up online.
>>
>> I see that right now rd.action_on_fail is parsed by emergency.service and
>> service does not start if this parameter is specified.
>>
>> Can't we interpret this parameter little differently. That is this
>> parameter modifies the OnFailure= behavior.
>>
>> So by default OnFailure= is emergency.service which is equivalent to
>> a shell.
>>
>> A user can force change of behavior by specifying command line.
>>
>> rd.action_on_failure=shell (OnFailure=emergency.service)
>> rd.action_on_failure=reboot (OnFailure=reboot)
>> rd.action_on_failure=continue (OnFailure=continue)
>>
>> Now action_on_failure=continue will effectively pretend that unit start
>> was successful and go ahead with starting next unit. This might be little
>> contentious though as other dependent units will fail in unknown ways.
>>
>> Now by default kdump can use rd.acton_on_failure=continue and try to
>> save dump. If it can't due to previous failures, then it will anyway
>> reboot the system.
>>
>> Also if emergency.service stops parsing rd.action_on_failure, then kdump
>> module will be able to start emergency.service too when it sees there
>> is a problem. Right now when kdump module tries to start emergency.service
>> it fails because it looks at acton_on_fail parameter (Another issue Bao is
>> trying to solve).
>>
>> Thanks
>> Vivek
>>
> 
> Why not install your own version of emergency.service in the kdump dracut
> module, which parses rd.action_on_failure and acts accordingly. Or replace
> emergency.service in the dracut cmdline hook according to rd.action_on_failure.

I was going to suggest this yesterday but as I'm not familiar with kdump
stuff I wasn't sure if I'd missed something, so kept quiet. Glad the
suggestion has come up now tho' :)

As for the 2nd option above, if emergency.service was replaced at
runtime it would require a systemctl daemon-reload which is probably not
idea. Maybe better to replace it statically in the kdump module as per
your first suggest and make it implement the logic needed at runtime
if/when it's called would be better.

Col

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Colin Guthrie
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