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

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Message-ID: <f95f2f30-1393-4ae1-96b1-96e4abfc368f@redhat.com>
Date: Fri, 30 May 2025 11:47:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Jiri Bohac <jbohac@...e.cz>
Cc: Michal Hocko <mhocko@...e.com>, Baoquan He <bhe@...hat.com>,
 Donald Dutile <ddutile@...hat.com>, 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 30.05.25 11:34, Jiri Bohac wrote:
> On Fri, May 30, 2025 at 11:11:40AM +0200, 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.
>>
>>> crashkernel=1G,cma # wait on transition with e.g. 10s timeout
>>
>> In general, would work for me.
> 
> I don't like extending the crashkernel= syntax like this.
> It would make hooking into the generic parsing code in
> parse_crashkernel() really ugly. The syntax is already
> convoluted as is and hard enough to explain in the documentation.

Would another boolean flag (on top of the other one you are adding) 
really make this significantly more ugly?

> 
> Also I don't see how adding a boolean knob is better than adding
> one that allows setting any arbitrary timeout. It has less
> flexibility and all the drawbacks of having an extra knob.

I guess Michals point is that specifying the higher-level problem and 
giving less flexibility mioght actually be less confusing for users.

> 
> I am inclined to just setting the fixed delay to 10s for now and
> adding a sysfs knob later if someone asks for it.
> 
> Would that work for you?

Sure. We could always add such a flag later if it's really a problem for 
someone.

-- 
Cheers,

David / dhildenb