linux-kernel - Re: [PATCH] mm/hotplug: Only respect mem= parameter during boot stage

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Date:   Tue, 10 Dec 2019 15:19:34 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, jgross@...e.com,
        william.kucharski@...cle.com, mingo@...nel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/hotplug: Only respect mem= parameter during boot stage

On Tue 10-12-19 22:05:34, Baoquan He wrote:
> On 12/10/19 at 02:32pm, Michal Hocko wrote:
> > On Tue 10-12-19 20:55:57, Baoquan He wrote:
> > [...]
> > > Btw, as you said at above, I am confused by the '[KNL,BOOT]', what does
> > > the 'BOOT' mean in the documentation of 'mem='? I checked all parameters
> > > with 'BOOT', still don't get it clearly.
> > 
> > This is a good question indeed. I have checked closer and this is what
> > documentation says
> > Documentation/admin-guide/kernel-parameters.rst
> > "
> >         BOOT    Is a boot loader parameter.
> > 
> > Parameters denoted with BOOT are actually interpreted by the boot
> > loader, and have no meaning to the kernel directly.
> > "
> > 
> > and that really doesn't fit, right? So I went to check the full history
> > git tree just to get to 2.4.0-test5 and no explanation whatsoever.
> > Fun, isn't it? ;)
> 
> Yeah, very interesting. Finally I got their original purpose from
> Documentation/x86/boot.rst.
> 
> 
> Special Command Line Options
> ============================
> 
> If the command line provided by the boot loader is entered by the
> user, the user may expect the following command line options to work.
> They should normally not be deleted from the kernel command line even
> though not all of them are actually meaningful to the kernel.  Boot
> loader authors who need additional command line options for the boot
> loader itself should get them registered in
> Documentation/admin-guide/kernel-parameters.rst to make sure they will not
> conflict with actual kernel options now or in the future.
> 
> ...
> 
> So here, [KNL,BOOT], KNL means it's used for kernel, BOOT means it's
> needed by boot loader.

OK, that clarifies this a bit. Thanks for referencing to it!
That should explain how the behavior is not boot time restricted at all
and the current implementation is actually correct. So a change to it
should clearly state the new usecase as we have already discussed. In
case there are bootloaders which really rely on the original strict
meaning then we should be able to compare cost/benfits of those two
usecases.
-- 
Michal Hocko
SUSE Labs