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

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Date:   Thu, 24 May 2018 11:41:58 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dave Young <dyoung@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Neil Horman <nhorman@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vivek Goyal <vgoyal@...hat.com>,
        Tony Luck <tony.luck@...el.com>,
        Anton Vorontsov <avorontsov@...mvista.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>, dzickus@...hat.com,
        bhe@...hat.com
Subject: Re: [PATCH] kdump: add default crashkernel reserve kernel config options

Dave Young <dyoung@...hat.com> writes:

> Hi Eric,
> On 05/23/18 at 10:53am, Eric W. Biederman wrote:
>> Dave Young <dyoung@...hat.com> writes:
>> 
>> > [snip]
>> >
>> >> >  
>> >> > +config CRASHKERNEL_DEFAULT_THRESHOLD_MB
>> >> > +	int "System memory size threshold for kdump memory default reserving"
>> >> > +	depends on CRASH_CORE
>> >> > +	default 0
>> >> > +	help
>> >> > +	  CRASHKERNEL_DEFAULT_MB is used as default crashkernel value if
>> >> > +	  the system memory size is equal or bigger than the threshold.
>> >> 
>> >> "the threshold" is rather vague.  Can it be clarified?
>> >> 
>> >> In fact I'm really struggling to understand the logic here....
>> >> 
>> >> 
>> >> > +config CRASHKERNEL_DEFAULT_MB
>> >> > +	int "Default crashkernel memory size reserved for kdump"
>> >> > +	depends on CRASH_CORE
>> >> > +	default 0
>> >> > +	help
>> >> > +	  This is used as the default kdump reserved memory size in MB.
>> >> > +	  crashkernel=X kernel cmdline can overwrite this value.
>> >> > +
>> >> >  config HAVE_IMA_KEXEC
>> >> >  	bool
>> >> >  
>> >> > @@ -143,6 +144,24 @@ static int __init parse_crashkernel_simp
>> >> >  	return 0;
>> >> >  }
>> >> >  
>> >> > +static int __init get_crashkernel_default(unsigned long long system_ram,
>> >> > +					  unsigned long long *size)
>> >> > +{
>> >> > +	unsigned long long sz = CONFIG_CRASHKERNEL_DEFAULT_MB;
>> >> > +	unsigned long long thres = CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB;
>> >> > +
>> >> > +	thres *= SZ_1M;
>> >> > +	sz *= SZ_1M;
>> >> > +
>> >> > +	if (sz >= system_ram || system_ram < thres) {
>> >> > +		pr_debug("crashkernel default size can not be used.\n");
>> >> > +		return -EINVAL;
>> >> 
>> >> In other words,
>> >> 
>> >> 	if (system_ram <= CONFIG_CRASHKERNEL_DEFAULT_MB ||
>> >> 	    system_ram < CONFIG_CRASHKERNEL_DEFAULT_THRESHOLD_MB)
>> >> 		fail;
>> >> 
>> >> yes?
>> >> 
>> >> How come?  What's happening here?  Perhaps a (good) explanatory comment
>> >> is needed.  And clearer Kconfig text.
>> >> 
>> >> All confused :(
>> >
>> > Andrew, I tuned it a bit, removed the check of sz >= system_ram, so if
>> > the size is too large and kernel can not find enough memory it will
>> > still fail in latter code.
>> >
>> > Is below version looks clearer?
>> 
>> What is the advantage of providing this in a kconfig option rather
>> than on the kernel command line as we can now?
>
> It is not a replacement of the cmdline, this can be a supplement to
> the crashkernel command line.  For a lot of common use cases if we have
> the auto reservation user just do not need to manually set the cmdline
> for example on a virtual machine and usual setup (except of the
> comlicate storage and very large machines).  The crashkernel=auto
> has been used for long time, Red Hat QE tested it on a lot of different
> lab machines and proved it works well.  Kdump usually just works so admin
> do little work to enable kdump.
>
> But the crashkernel=auto implementation has some drawbacks that is it
> is more like embed policy in the code and it is not flexible like a
> config option.

Have you considered using the builtin command line aka CONFIG_CMDLINE?
If as you are reserving a fixed amount of memory as your patch does that
should be sufficient, and doable without any kernel changes.

Eric