linux-kernel - Re: [PATCH v5 0/5] kdump: crashkernel reservation from CMA

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Message-ID: <2e35b6dd-56dd-47e6-8dac-54f446f763f0@redhat.com>
Date: Mon, 6 Oct 2025 18:45:10 +0200
From: David Hildenbrand <dhildenb@...hat.com>
To: Breno Leitao <leitao@...ian.org>, kas@...nel.org
Cc: Jiri Bohac <jbohac@...e.cz>, riel@...riel.com, vbabka@...e.cz,
 nphamcs@...il.com, Baoquan He <bhe@...hat.com>,
 Vivek Goyal <vgoyal@...hat.com>, Dave Young <dyoung@...hat.com>,
 kexec@...ts.infradead.org, akpm@...ux-foundation.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 v5 0/5] kdump: crashkernel reservation from CMA

On 06.10.25 18:25, Breno Leitao wrote:
> On Mon, Oct 06, 2025 at 10:16:26AM +0200, David Hildenbrand wrote:
>> On 03.10.25 17:51, Breno Leitao wrote:
>>> Hello Jiri,
>>>
>>> On Thu, Jun 12, 2025 at 12:11:19PM +0200, Jiri Bohac wrote:
>>>
>>>> Currently this is only the case for memory ballooning and zswap. Such movable
>>>> memory will be missing from the vmcore. User data is typically not dumped by
>>>> makedumpfile.
>>>
>>> For zswap and zsmalloc pages, I'm wondering whether these pages will be missing
>>> from the vmcore, or if there's a possibility they might be present but
>>> corrupted—especially since they could reside in the CMA region, which may be
>>> overwritten by the kdump environment.
>>
>> That's not different to ordinary user pages residing on these areas, right?
> 
> Will zsmalloc on CMA pages be marked as "userpages"?

No, but they should have the zsmalloc page type set.

> 
> makedump file iterates over the pfns and check for a few flags before
> "copying" them to disk.
> 
> In makedumpfile, userpages are basically discarded if they are anonymous
> pages:
> 	#define isAnon(mapping, flags, _mapcount) \
> 		(((unsigned long)mapping & PAGE_MAPPING_ANON) != 0 && !isSlab(flags,
> 		_mapcount))
> 
> 	https://github.com/makedumpfile/makedumpfile/blob/master/makedumpfile.h#L164
> 
> 	called from:
> 	https://github.com/makedumpfile/makedumpfile/blob/master/makedumpfile.c#L6671
> 
> For zsmalloc pages in the CMA, The page struct (pfn)) is marked with old
> page struct (from the first kernel), but, the content has changed
> (replaced by kdump environment - 2nd kernel).
> 
> So, whatever decision makedumpfile does based on the PFN, it will dump
> incorrect data, given that the page content does not match the data
> anymore.

Right.

> 
> If my understanding is valid, we don't want to dump any page that points
> to the PFN, because they will probably have garbage.

My theory is that barely anybody will go ahead and check compressed page 
content, but I agree. We should filter them out.

> 
> That said, I see two options:
> 
>   1) Ignore the CMA area completely in makedump.
>      - I don't think there is any way to find that area today. The kernel
>        might need to print the CMA region somewhere (/proc/iomem?)

/proc/iomem in the newkernel should indicate the memory region as System 
RAM (for the new kernel). That can just be filtered out in any case: 
dumping memory of the new kernel does not make sense in any case.

> 
>   2) Given that most of the memory in CMA will be anonymous memory, and
>      already discard by other rules, just add an additional entry for
>      zsmalloc pages.
> 
>      Talking to Kirill offline, it seems we can piggy back on MovableOps
>      page flag.

We should likely check the page type instead if we go down that path.

-- 
Cheers

David / dhildenb