Re: [PATCH v3 2/2] mm: Introduce kernelcore=mirror option

From: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: "Luck, Tony" <tony.luck@intel.com>, Xishi Qiu <qiuxishi@huawei.com>
Cc: "Izumi, Taku" <izumi.taku@jp.fujitsu.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"mel@csn.ul.ie" <mel@csn.ul.ie>,
	"Hansen, Dave" <dave.hansen@intel.com>,
	"matt@codeblueprint.co.uk" <matt@codeblueprint.co.uk>
Subject: Re: [PATCH v3 2/2] mm: Introduce kernelcore=mirror option
Date: Fri, 18 Dec 2015 11:12:13 +0900	[thread overview]
Message-ID: <56736B7D.3040709@jp.fujitsu.com> (raw)
In-Reply-To: <3908561D78D1C84285E8C5FCA982C28F39F882E8@ORSMSX114.amr.corp.intel.com>

On 2015/12/18 3:43, Luck, Tony wrote:
>>>> As Tony requested, we may need a knob to stop a fallback in "movable->normal", later.
>>>>
>>>
>>> If the mirrored memory is small and the other is large,
>>> I think we can both enable "non-mirrored -> normal" and "normal -> non-mirrored".
>>
>> Size of mirrored memory can be configured by software(EFI var).
>> So, having both is just overkill and normal->non-mirroed fallback is meaningless considering
>> what the feature want to guarantee.
>
> In the original removable usage we wanted to guarantee that Linux did not allocate any
> kernel objects in removable memory - because that would prevent later removal of that
> memory.
>
> Mirror case is the same - we don't want to allocate kernel structures in non-mirrored memory
> because an uncorrectable error in one of them would crash the system.
>
> But I think some users might like some flexibility here.  If the system doesn't have enough
> memory for the kernel (non-movable or mirrored), then it seems odd to end up crashing
> the system at the point of memory exhaustion (a likely result ... the kernel can try to reclaim
> some pages from SLAB, but that might only return a few pages, if the shortage continues
> the system will perform poorly and eventually fail).
>
> The whole point of removable memory or mirrored memory is to provide better availability.
>
> I'd vote for a mode where running out of memory for kernel results in a
>
>     warn_on_once("Ran out of mirrored/non-removable memory for kernel - now allocating from all zones\n")
>
> because I think most people would like the system to stay up rather than worry about some future problem that may never happen.

Hmm...like this ?
       sysctl.vm.fallback_mirror_memory = 0  // never fallback  # default.
       sysctl.vm.fallback_mirror_memory = 1  // the user memory may be allocated from mirrored zone.
       sysctl.vm.fallback_mirror_memory = 2  // usually kernel allocates memory from mirrored zone before OOM.
       sysctl.vm.fallback_mirror_memory = 3  // 1+2

However I believe my customer's choice is always 0, above implementation can be done in a clean way.
(adding a flag to zones (mirrored or not) and controlling fallback zonelist walk.)

BTW, we need this Taku's patch to make a progress. I think other devs should be done in another
development cycle. What does he need to get your Acks ?

Thanks,
-Kame