Re: [PATCH v9 0/8] kdump: reduce vmcore size and capture time

[Wandun <chenwandun1@gmail.com>]

On 10/21/26 16:30, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
> 
> 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.
> 
> ACPI systems already filter reserved memory out of the vmcore through
> their existing path: firmware reports those carveouts as unusable in
> the EFI memory map, they get MEMBLOCK_NOMAP set in memblock.memory
> (is_usable_memory(), efi-init.c), and the ELF header builder has
> always skipped nomap regions; only DT-based systems currently fail
> to filter these regions, which is what this series addresses.
> 
> This series introduces a MEMBLOCK_NODUMP flag (suggested by Rob) in
> memblock to filter reserved memory on DT-based architectures. A new
> flag is needed because MEMBLOCK_NOMAP can only express regions without
> a linear mapping, while MEMBLOCK_NODUMP can filter regions that are
> linearly mapped. 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.
> 
> 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 4-7: 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 entries with MEMBLOCK_NODUMP.
> Patch 8:     Skip MEMBLOCK_NODUMP regions when building the vmcore
>              ELF header.

Hi all,

A gentle ping on this series.

I'd appreciate any further feedback, particularly on the
MEMBLOCK_NODUMP discussion and the issues raised in the Sashiko
review.

Please let me know if there are any other concerns or suggestions
before I prepare the next revision.

Thanks,
Wandun

> 
> v8 --> v9:
> 1. Patch 7: Avoid a potential boot panic on SH when NODUMP marking requires
>    memblock resizing before it is allowed, found by Sashiko in [1]. SH supports
>    neither CONFIG_KEXEC_FILE nor CONFIG_CRASH_HOTPLUG, so skip marking when
>    both are disabled.
> 
> v6 --> v8 (v7 mainly tried to fix the initrd issue on release in v6,
> that approach is superseded here):
> 1. Stop marking /memreserve/ regions with NODUMP. v6 marked
>    /memreserve/ regions with NODUMP. But a bootloader may specify
>    a /memreserve/ entry for memory that the kernel later frees
>    (e.g. the initrd), and those pages would then be wrongly
>    filtered out of the vmcore after being reused as normal memory.
>    The v7 fix (clearing the flag in free_reserved_area()) caused
>    a lockless runtime mutation of memblock.memory [2]. Better solutions
>    exist but need extra preparation, so temporarily dropped the patch
>    that introduced the problem to keep this series simple in v8.
> 2. Fix a build failure in patch 2 with both ARCH_KEEP_MEMBLOCK and
>    MEMORY_HOTPLUG are unset. There the memblock arrays live in
>    .init.data, and the weak defaults referencing them create section
>    mismatches, which turn into build errors when SECTION_MISMATCH_WARN_ONLY
>    is also unset.
> 3. RISCV recently gained crash hotplug support, so adapt its handler
>    like x86 in patch 3: use __crash_prepare_headers() instead of
>    crash_prepare_headers() to avoid self-deadlock.
> 4. Update the cover letter per review feedback: describe how ACPI
>    systems filter reserved memory and why a new flag is needed
>    instead of reusing NOMAP.
> 
> v6 --> v7:
> 1. Fix a stale MEMBLOCK_NODUMP flag issue [3]. 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.
> 
> [1] https://sashiko.dev/#/patchset/20261016061549.4181390-1-chenwandun1@gmail.com?part=7
> [2] https://lore.kernel.org/lkml/20260922090252.B01541F000FF@smtp.kernel.org/
> [3] https://sashiko.dev/#/message/20260917220750.GA4013814-robh%40kernel.org
> 
> v5: https://lore.kernel.org/lkml/20260806093519.2907771-1-chenwandun1@gmail.com/
> v4: https://lore.kernel.org/lkml/20260630074715.4126796-1-chenwandun1@gmail.com/
> 
> Wandun Chen (8):
>   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
>   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/crash.c                  |  2 +-
>  arch/riscv/kernel/machine_kexec_file.c     | 33 --------------
>  arch/x86/kernel/crash.c                    |  2 +-
>  drivers/of/fdt.c                           |  1 +
>  drivers/of/of_private.h                    |  1 +
>  drivers/of/of_reserved_mem.c               | 18 ++++++++
>  include/linux/crash_core.h                 |  2 +
>  include/linux/memblock.h                   |  9 ++++
>  include/linux/of_reserved_mem.h            |  1 +
>  kernel/crash_core.c                        | 53 +++++++++++++++++++++-
>  kernel/dma/contiguous.c                    |  1 +
>  mm/memblock.c                              | 16 +++++++
>  15 files changed, 103 insertions(+), 94 deletions(-)
>