linux-kernel - Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

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Date:   Fri, 1 Dec 2023 16:51:13 +0100
From:   Philipp Rudo <prudo@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Baoquan He <bhe@...hat.com>, Donald Dutile <ddutile@...hat.com>,
        Jiri Bohac <jbohac@...e.cz>, Pingfan Liu <piliu@...hat.com>,
        Tao Liu <ltao@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

On Fri, 1 Dec 2023 12:55:52 +0100
Michal Hocko <mhocko@...e.com> wrote:

> On Fri 01-12-23 12:33:53, Philipp Rudo wrote:
> [...]
> > And yes, those are all what-if concerns but unfortunately that is all
> > we have right now.  
> 
> Should theoretical concerns without an actual evidence (e.g. multiple
> drivers known to be broken) become a roadblock for this otherwise useful
> feature? 

Those concerns aren't just theoretical. They are experiences we have
from a related feature that suffers exactly the same problem regularly
which wouldn't exist if everybody would simply work "properly".

And yes, even purely theoretical concerns can become a roadblock for a
feature when the cost of those theoretical concerns exceed the benefit
of the feature. The thing is that bugs will be reported against kexec.
So _we_ need to figure out which of the shitty drivers caused the
problem. That puts additional burden on _us_. What we are trying to
evaluate at the moment is if the benefit outweighs the extra burden
with the information we have at the moment.

> > Only alternative would be to run extended tests in
> > the field. Which means this user facing change needs to be included.
> > Which also means that we are stuck with it as once a user facing change
> > is in it's extremely hard to get rid of it again...  
> 
> I am not really sure I follow you here. Are you suggesting once
> crashkernel=cma is added it would become a user api and therefore
> impossible to get rid of?

Yes, sort of. I wouldn't rank a command line parameter as user api. So
we still can get rid of it. But there will be long discussions I'd like
to avoid if possible.

Thanks
Philipp