linux-kernel - Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

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Message-ID: <aDl5rpqCUyf7nX2M@tiehlicka>
Date: Fri, 30 May 2025 11:26:06 +0200
From: Michal Hocko <mhocko@...e.com>
To: David Hildenbrand <david@...hat.com>
Cc: Baoquan He <bhe@...hat.com>, Donald Dutile <ddutile@...hat.com>,
	Jiri Bohac <jbohac@...e.cz>, Vivek Goyal <vgoyal@...hat.com>,
	Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
	Philipp Rudo <prudo@...hat.com>, Pingfan Liu <piliu@...hat.com>,
	Tao Liu <ltao@...hat.com>, linux-kernel@...r.kernel.org,
	David Hildenbrand <dhildenb@...hat.com>
Subject: Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

On Fri 30-05-25 11:11:40, David Hildenbrand wrote:
> On 30.05.25 11:07, Michal Hocko wrote:
> > On Fri 30-05-25 10:39:39, David Hildenbrand wrote:
> > > On 30.05.25 10:28, Michal Hocko wrote:
> > [...]
> > > > All that being said I would go with an additional parameter to the
> > > > kdump cma setup - e.g. cma_sane_dma that would skip waiting and use 10s
> > > > otherwise. That would make the optimized behavior opt in, we do not need
> > > > to support all sorts of timeouts and also learn if this is not
> > > > sufficient.
> > > > 
> > > > Makes sense?
> > > 
> > > Just so I understand correctly, you mean extending the "crashkernel=" option
> > > with a boolean parameter? If set, e.g., wait 1s, otherwise magic number 10?
> > 
> > crashkernel=1G,cma,cma_sane_dma # no wait on transition
> 
> But is no wait ok? I mean, any O_DIRECT with any device would at least take
> a bit, no?
> 
> Of course, there is a short time between the crash and actually triggerying
> kdump.

This is something we can test for and if we need a short timeout in this
case as well then it is just trivial to add it. I am much more
concerned about those potentially unpredictable DMA transfers that could
take too long and it is impossible to test for those and therefore we
need to overshoot.
 
> > crashkernel=1G,cma # wait on transition with e.g. 10s timeout
> 
> In general, would work for me.
> 
> -- 
> Cheers,
> 
> David / dhildenb

-- 
Michal Hocko
SUSE Labs