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

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Message-ID: <aOSO/e68pZ7FGKm1@MiWiFi-R3L-srv>
Date: Tue, 7 Oct 2025 11:55:36 +0800
From: Baoquan He <bhe@...hat.com>
To: David Hildenbrand <dhildenb@...hat.com>,
	Breno Leitao <leitao@...ian.org>
Cc: kas@...nel.org, Jiri Bohac <jbohac@...e.cz>, riel@...riel.com,
	vbabka@...e.cz, nphamcs@...il.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 10/06/25 at 06:45pm, David Hildenbrand wrote:
> 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.

Agree.

And I saw Jiri has excluded the crashk_cma_ranges[] from the dumped
content via elf_header_exclude_ranges(). Have you encountered a real
problem about the dumping, or you are just worried about it?

> 
> > 
> >   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.

Talking about the pages in CMA except of crashk_cma_ranges[],
zsmalloc/zswap is true as anon mem and can be discarded. I am wondering
if there's any driver or kernel pages residing in CMA and being worth to
dump out.