linux-kernel - Re: [PATCH v3 3/5] kdump, documentation: describe craskernel CMA reservation

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Message-ID: <c5495dd1-8085-4721-af1e-93f34f36d97b@redhat.com>
Date: Fri, 27 Jun 2025 14:18:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Baoquan He <bhe@...hat.com>, Jiri Bohac <jbohac@...e.cz>,
 David Hildenbrand <dhildenb@...hat.com>
Cc: akpm@...ux-foundation.org, Vivek Goyal <vgoyal@...hat.com>,
 Dave Young <dyoung@...hat.com>, kexec@...ts.infradead.org,
 Philipp Rudo <prudo@...hat.com>, Donald Dutile <ddutile@...hat.com>,
 Pingfan Liu <piliu@...hat.com>, Tao Liu <ltao@...hat.com>,
 linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v3 3/5] kdump, documentation: describe craskernel CMA
 reservation

On 27.06.25 14:16, David Hildenbrand wrote:
> On 14.03.25 04:18, Baoquan He wrote:
>> Hi Jiri,
>>
>> On 03/12/25 at 10:09pm, Jiri Bohac wrote:
>> ......
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index fb8752b42ec8..895b974dc3bb 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -987,6 +987,28 @@
>>>    			0: to disable low allocation.
>>>    			It will be ignored when crashkernel=X,high is not used
>>>    			or memory reserved is below 4G.
>>> +	crashkernel=size[KMG],cma
>>> +			[KNL, X86] Reserve additional crash kernel memory from
>>> +			CMA. This reservation is usable by the first system's
>>> +			userspace memory and kernel movable allocations (memory
>>> +			balloon, zswap). Pages allocated from this memory range
>>> +			will not be included in the vmcore so this should not
>>> +			be used if dumping of userspace memory is intended and
>>> +			it has to be expected that some movable kernel pages
>>> +			may be missing from the dump.
>>
>> Since David and Don expressed concern about the missing kernel pages
>> allocated from CMA area in v2, and you argued this is still useful for
>> VM system, I would like to invite David to help evaluate the whole
>> series if it's worth from the VM and MM point of view.
> 
> Balloon pages will not be dumped either way (PageOffline), so that is
> not a convern.
> 
> Zsmalloc pages ... are probably fine right now. They should likely only
> be storing compressed user data. (not sure if they also store some other
> datastructures, I think no, but might be wrong)
> 
> My comment was rather forward-looking: that CMA memory only contains
> user space memory is already not the case (but the existing cases might
> be okay). In the future, as we support other movable allocations (as
> raised, leaf page tables at some point, and there were discussions about
> movable slab pages, although that might be challenging) this can change
> (unless we find ways of not placing these allocations on CMA memory).
> 
> So as is, this should be fine, but it's certainly something to be aware
> of in the future.
> 

BTW, I realize this was a late reply, and that the series already 
proceeded. Just stumbled over that un-replied mail an thought I'd 
clarify my point here.

-- 
Cheers,

David / dhildenb