Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

From: Baoquan He <bhe@redhat.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Philipp Rudo <prudo@redhat.com>,
	Donald Dutile <ddutile@redhat.com>, Jiri Bohac <jbohac@suse.cz>,
	Pingfan Liu <piliu@redhat.com>, Tao Liu <ltao@redhat.com>,
	Vivek Goyal <vgoyal@redhat.com>, Dave Young <dyoung@redhat.com>,
	kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
	David Hildenbrand <dhildenb@redhat.com>
Subject: Re: [PATCH 0/4] kdump: crashkernel reservation from CMA
Date: Fri, 8 Dec 2023 10:10:46 +0800	[thread overview]
Message-ID: <ZXJ7JpcZALSeFvox@MiWiFi-R3L-srv> (raw)
In-Reply-To: <ZXGIeAgCcatUDa2h@tiehlicka>

On 12/07/23 at 09:55am, Michal Hocko wrote:
> On Thu 07-12-23 12:23:13, Baoquan He wrote:
> [...]
> > We can't guarantee how swift the DMA transfer could be in the cma, case,
> > it will be a venture.
> 
> We can't guarantee this of course but AFAIK the DMA shouldn't take
> minutes, right? While not perfect, waiting for some time before jumping
> into the crash kernel should be acceptable from user POV and it should
> work around most of those potential lingering programmed DMA transfers.
> 
> So I guess what we would like to hear from you as kdump maintainers is
> this. Is it absolutely imperative that these issue must be proven
> impossible or is a best effort approach something worth investing time
> into? Because if the requirement is an absolute guarantee then I simply
> do not see any feasible way to achieve the goal of reusable memory.

Honestly, I think all the discussions and proof have told clearly it's
not a good idea. This is not about who wants this, who doesn't. So
far, this is an objective fact that taking ,cma area for crashkernel= is
not a good idea, it's very risky.

We don't deny this at the beginning. I tried to present all what I know,
we have experienced, we have investigated, we have tried. I wanted to
see if this time we can clarify some concerns may be mistaken. But it's
not. The risk is obvious and very likely happen.

> 
> Let me reiterate that the existing reservation mechanism is showing its
> limits for production systems and I strongly believe this is something
> that needs addressing because crash dumps are very often the only tool
> to investigate complex issues.

Yes, I admit that. But it haven't got to the point that it's too bad to
bear so that we have to take the risk to take ,cma instead.

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