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

Baoquan He
2025-10-07T03:55:55Z

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.