Re: [PATCH v7 0/9] kdump: reduce vmcore size and capture time

[Wandun]

On 10/2/26 16:17, Mike Rapoport wrote:
> Hi Wandun,
> 
> On Tue, Sep 22, 2026 at 04:42:47PM +0800, Wandun Chen wrote:
>> From: Wandun Chen <chenwandun@xxxxxxxxxxx>
>>
>> On SoCs that carve out large firmware-owned reserved memory (GPU,
>> camera ISP, ...), kdump currently dumps those carveouts as part of
>> system RAM even though their contents are firmware state that is not
>> useful for kernel crash analysis.
>>
>> This series introduces a MEMBLOCK_NODUMP flag in memblock to filter
>> reserved memory on DT-based architectures (arm64, riscv, loongarch).
>> Reserved regions are marked MEMBLOCK_NODUMP by default so kdump omits
>> them; reusable CMA regions are different because their pages are handed
>> back to the buddy allocator and may carry crash-relevant data.
>>
>> ACPI systems already filter reserved memory out of the vmcore through
>> their existing path; only DT-based systems currently fail to filter these
>> regions, which is what this series addresses. The flag lives in memblock
>> itself rather than in a DT-only structure, so both ACPI and DT systems
>> can benefit from it (suggested by Rob, thanks).
> 
Hi Mike,

Thanks for the review.

> Can you elaborate how ACPI systems filter their reserved memory?

In the EFI memory map, firmware carveouts are reported with the type
EFI_RESERVED_TYPE. This type of memory gets the nomap flag set in
memblock.memory (is_usable_memory() returns false in reserve_regions()
and mark these regions with nomap flag), during prepare_elf_headers(),
for_each_mem_range() filters out these nomap regions. Therefore, the
memory reserved on ACPI based systems will not be dumped into the vmcore.

I will add these description in next version.

> With your patches, the memblock flag becomes a part of DT-only
> infrastructure, so that claim that both ACPI and DT benefit from it does
> not hold, doesn't it?

You are right, the wording was misleading. Let me clarify what I meant.

In this series only DT marks regions as MEMBLOCK_NODUMP:
/reserved-memory and /memreserve/ entries are tagged in the OF
paths (of_reserved_mem.c, fdt.c). ACPI does not set the flag and
keeps filtering its reserved memory through the existing path, as
described above.


What I intended to convey is that the flag itself lives in memblock
rather than on the DT-only struct reserved_mem, so it is available
for reuse. If an ACPI system ever needed to exclude a firmware-owned
region from the vmcore, it could mark that region with MEMBLOCK_NODUMP
the same way DT does, instead of having to invent a parallel mechanism.
That is a future option, not something this series enables for ACPI
today.

Also will update these description in next verison.

Best regards,
Wandun

>  
>> Since the reserved memory regions are filtered out, the vmcore is
>> smaller in size and faster to produce.
>>
>> The series is based on linux-next and is organized as follows:
>>
>> Patches 1-3: Preparation and bugfixes: switch riscv crash_mem to memblock,
>> 	     fold the duplicated per-arch memblock walks into the weak
>>              defaults, and serialize crash header preparation against
>>              memory hotplug.
>> Patches 4-8: NODUMP infrastructure: switch crash_core to
>>              for_each_mem_region(), introduce the MEMBLOCK_NODUMP flag,
>> 	     add a dumpable flag to struct reserved_mem, and mark
>> 	     /reserved-memory and /memreserve/ entries with MEMBLOCK_NODUMP
>> 	     flag.
>> Patch 9:     Exclude MEMBLOCK_NODUMP regions from the vmcore ELF
>>              header.
>>
>> v6 --> v7:
>> 1. Fix a stale MEMBLOCK_NODUMP flag issue [1]. Memory reserved and
>>    marked as NODUMP may later be freed (the initrd, for example),
>>    so the NODUMP flag must be cleared accordingly. Since the fix is
>>    part of the NODUMP infrastructure introduced by patch 5, it has
>>    been folded into that patch.
>>
>> 2. Drop the "generic" related description in the cover letter.
>>
>> 3. Collect the Acked-by and Reviewed-by tags received during the v6
>>    review (thanks to Baoquan, Mike, Rob and Marek). Patch 5 is the
>>    only one without a tag, as its Reviewed-by was dropped when the
>>    fix was folded in.
>>
>> 4. Drop patch 1 of v6, which has been merged separately.
>>
>>
>> v5 --> v6:
>> 1. Serialize crash header preparation against memory hotplug to avoid
>>    out-of-bounds or use-after-free issues.
>> 2. MEMBLOCK_NODUMP marking is now done after memblock allows resizing,
>>    avoiding a panic from too few regions before resize is permitted.
>> 3. Reordered the patches, put pre-existing bugfixes earlier in the series.
>>
>>
>> v4 --> v5:
>> 1. Rework the mechanism around a memblock-level MEMBLOCK_NODUMP flag
>>    (suggested by Rob) instead of the v4 opt-in 'dumpable' flag on
>>    DT-only struct reserved_mem [2].
>> 2. Switch the riscv vmcore elf header preparation to use memblock
>>    instead of the resource tree, aligning it with arm64 and loongarch,
>>    so riscv also can exclude reserved memory from vmcore.
>> 3. Deduplicate the vmcore elf header preparation: arm64, riscv and
>>    loongarch open-coded the same logic, so fold it into shared
>>     __weak defaults in crash_core.
>> 4.  Drop the v4 patch that saved /memreserve/ entries into the
>>     reserved_mem array; /memreserve/ is now marked MEMBLOCK_NODUMP
>>     directly.
>>
>> v3 --> v4:
>> 1. Rebase this series on v7.2-rc1.
>> 2. Add two cleanup patches (patch 02/03).
>> 3. Simplify patch 03 to avoid checking whether initial_boot_params is
>>    NULL multiple times, suggested by Rob.
>>
>> v2 --> v3:
>> 1. Fix out-of-bounds issue if device tree lacks /reserved-memory node.
>> 2. Fix UAF issue when alloc_reserved_mem_array() fails.
>> 3. Add some prepare patches.
>>
>> v1 --> v2:
>> 1. v1 added an opt-out DT property ('linux,no-dump'). Per Rob's
>>    feedback [3], v2 drop that property and exclude reserve memory
>>    by default.
>> 2. Split some prepared patches from the original patches.
>> 3. Address coding-style comments on patch 5 from Rob.
>>
>> [1] https://sashiko.dev/#/message/20260917220750.GA4013814-robh%40kernel.org
>> [2] https://lore.kernel.org/lkml/20260723234126.GA3253409-robh@xxxxxxxxxx/
>> [3] https://lore.kernel.org/lkml/20260506144542.GA2072596-robh@xxxxxxxxxx/
>>
>>
>>
>> Wandun Chen (9):
>>   riscv: build crash_mem ranges from memblock instead of resource tree
>>   crash_core: fold duplicated memblock arch hooks into the weak default
>>   crash_core: serialize crash header preparation against hotplug
>>   crash_core: replace for_each_mem_range() with for_each_mem_region()
>>   memblock: introduce MEMBLOCK_NODUMP flag
>>   of: reserved_mem: add dumpable flag to opt-in vmcore
>>   of: reserved_mem: mark /reserved-memory entries with MEMBLOCK_NODUMP
>>   of: reserved_mem: mark /memreserve/ entries as MEMBLOCK_NODUMP
>>   crash_core: skip MEMBLOCK_NODUMP regions when building vmcore ELF
>>     header
>>
>>  arch/arm64/kernel/machine_kexec_file.c     | 29 ------------
>>  arch/loongarch/kernel/machine_kexec_file.c | 27 ------------
>>  arch/riscv/Kconfig                         |  2 +-
>>  arch/riscv/kernel/machine_kexec_file.c     | 33 --------------
>>  arch/x86/kernel/crash.c                    |  2 +-
>>  drivers/of/fdt.c                           |  2 +
>>  drivers/of/of_private.h                    |  2 +
>>  drivers/of/of_reserved_mem.c               | 48 ++++++++++++++++++++
>>  include/linux/crash_core.h                 |  2 +
>>  include/linux/memblock.h                   |  9 ++++
>>  include/linux/of_reserved_mem.h            |  1 +
>>  kernel/crash_core.c                        | 51 ++++++++++++++++++++--
>>  kernel/dma/contiguous.c                    |  1 +
>>  mm/memblock.c                              | 43 +++++++++++++++++-
>>  14 files changed, 155 insertions(+), 97 deletions(-)
>>
>> -- 
>> 2.43.0
>>
>