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

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Message-ID: <ZXGIeAgCcatUDa2h@tiehlicka>
Date:   Thu, 7 Dec 2023 09:55:20 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Philipp Rudo <prudo@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Jiri Bohac <jbohac@...e.cz>, Pingfan Liu <piliu@...hat.com>,
        Tao Liu <ltao@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        David Hildenbrand <dhildenb@...hat.com>
Subject: Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

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.

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.
-- 
Michal Hocko
SUSE Labs