linux-kernel - Re: [PATCH] kdump: add default crashkernel reserve kernel config options

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Date:   Thu, 24 May 2018 16:29:51 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Dave Young <dyoung@...hat.com>
Cc:     Petr Tesarik <ptesarik@...e.cz>, dzickus@...hat.com,
        Neil Horman <nhorman@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] kdump: add default crashkernel reserve kernel config
 options

On 05/24/18 at 03:56pm, Dave Young wrote:
> > > > Instead of setting aside a significant chunk of memory nobody can use,
> > > > [...] reserve a significant chunk of memory that the kernel is prevented
> > > > from using [...], but applications are free to use it.
> > > 
> > > That works great, because user space pages are filtered out in the
> > > common case, so they can be used freely by the panic kernel.
> > 
> > Good suggestion. I have been reading that posts already at the same time before I saw
> > this reply from you :)
> > 
> > That could be a good idea and worth to discuss more.  I cced Hari
> > already in the thread. Hari, is it possible for you to extend your
> > idea to general use, ie. shared by both kdump and fadump?  Anyway I
> > think that is another topic we can discuss separately.
> 
> BTW, I remember we had some Red hat internal discussion about CMA previously
> there is a problem, that is we have crashkernel=,high for reserving high
> memory and ,low for low memory, we were not sure if CMA can handle this
> case.

Pratyush ever investigated this too see if CMA can be used so that we
can dynamically adjust the crashkernel memory after boot. The result is
CMA is not good. I doubt fadump can really make use of CMA to reserve
crashkernel.

Thanks
Baoquan