Re: [PATCH part5 0/7] Arrange hotpluggable memory as ZONE_MOVABLE.

From: KOSAKI Motohiro
Date: Wed Aug 14 2013 - 21:09:02 EST

(8/14/13 5:36 PM), Tejun Heo wrote:
> On Wed, Aug 14, 2013 at 05:17:23PM -0400, KOSAKI Motohiro wrote:
>> You haven't explain practical benefit of your opinion. As far as users have
>> no benefit, I'm never agree. Sorry.
> Umm... how about being more robust and actually useable to begin with?
> What's the benefit of panicking? Are you seriously saying that the
> admin / boot script can use the kernel boot param to tell the kernel
> to enable hotplug but can't check what nodes are hot unpluggable
> afterwards? The admin *needs* to check which nodes are hotpluggable
> no matter how this part is handled. How else is it gonna know which
> nodes are hotpluggable? Magic?
> There's no such rule as kernel param should make the kernel panic if
> it's not happy, so please take that out of your brain. It of course
> should be clear what the result of the kernel parameter is and
> panicking is the crudest way to do that which is good enough or even
> desriable in *some* cases. It is not the required behavior by any
> stretch of imgination, especially when the result of the parameter may
> change due to changing circumstances. That's an outright idiotic
> thing to do.

Sigh, I'd like to point a link of past discussion. But I can't find it now.
Let's summarize past discussion as far as possible.

Firstly, technically you can't implement correct fallback. You used a term
"when can't allocate memory", but it's not so simple. Think following scenario,
memory is enough for kernel image, but kernel will load memory hogging drivers.
The system will crash after boot within 1 min. Then, MM subsystem don't believe
a fallback. Bogus and misguided fallback give a user false relief and they don't
notice their mistake quickly. The answer is, there is the fundamental rule.
We always said, "measure your system carefully, and setting option carefully too".
I have no seen any reason to make exception in this case.

Secondly, memory hotplug is now maintained I and kamezawa-san. Then, I much likely
have a chance to get a hotplug related bug report. For protecting my life, I don't
want get a false bug claim. Then, I wouldn't like to aim incomplete fallback. When
an admin makes mistake, they should shoot their foot, not me!

Thirdly, I haven't insist to aim verbose and kind messages as last breath. It much
likely help users.

Last, we are now discussing hotplug feature. Then, we can assume hotpluggable machine.
They have a hotplug interface in farmware by definition. So, you need to aim a magic.

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