| Message ID | 20230407011507.17572-1-bhe@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | arm64: kdump : take off the protection on crashkernel memory region | expand |
On Fri, 7 Apr 2023 09:15:04 +0800, Baoquan He wrote: > Problem: > ======= > On arm64, block and section mapping is supported to build page tables. > However, currently it enforces to take base page mapping for the whole > linear mapping if CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabled and > crashkernel kernel parameter is set. This will cause longer time of the > linear mapping process during bootup and severe performance degradation > during running time. > > [...] Applied to arm64 (for-next/kdump), thanks! [1/3] arm64: kdump : take off the protection on crashkernel memory region https://git.kernel.org/arm64/c/0d124e96051b [2/3] arm64: kdump: do not map crashkernel region specifically https://git.kernel.org/arm64/c/04a2a7af3d97 [3/3] arm64: kdump: defer the crashkernel reservation for platforms with no DMA memory zones https://git.kernel.org/arm64/c/504cae453f82 Cheers,
Problem: ======= On arm64, block and section mapping is supported to build page tables. However, currently it enforces to take base page mapping for the whole linear mapping if CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabled and crashkernel kernel parameter is set. This will cause longer time of the linear mapping process during bootup and severe performance degradation during running time. Root cause: ========== On arm64, crashkernel reservation relies on knowing the upper limit of low memory zone because it needs to reserve memory in the zone so that devices' DMA addressing in kdump kernel can be satisfied. However, the upper limit of low memory on arm64 is variant. And the upper limit can only be decided late till bootmem_init() is called [1]. And we need to map the crashkernel region with base page granularity when doing linear mapping, because kdump needs to protect the crashkernel region via set_memory_valid(,0) after kdump kernel loading. However, arm64 doesn't support well on splitting the built block or section mapping due to some cpu reststriction [2]. And unfortunately, the linear mapping is done before bootmem_init(). To resolve the above conflict on arm64, the compromise is enforcing to take base page mapping for the entire linear mapping if crashkernel is set, and CONFIG_ZONE_DMA or CONFIG_ZONE_DMA32 is enabed. Hence performance is sacrificed. Solution: ========= Comparing with the always encountered base page mapping for the whole linear region, it's better to take off the protection on crashkernel memory region for now because the protection can only happen in a chance in one million, while the base page mapping for the whole linear mapping is always mitigating arm64 systems with crashkernel set. This can let distros have chance to back port this patchset to fix the performance issue caused by the base page mapping in the whole linear region. TODO: ====== Add purgatory to kexec_file_load interface on arm64, then checksum verification can be done there to check if stamping on crashkernel memory region happened. v1->v2: - When trying to revert commit 031495635b46, two hunks were missed in v1 post. Remove them in v2. Thanks to Leizhen for pointing out this. - Remove code comment above arm64_dma_phys_limit definition added in commit 031495635b46; - Move the arm64_dma_phys_limit assignment back into zone_sizes_init() when both CONFIG_ZONE_DMA and CONFIG_ZONE_DMA32 are not enabled. [1] https://lore.kernel.org/all/YrIIJkhKWSuAqkCx@arm.com/T/#u [2] https://lore.kernel.org/linux-arm-kernel/20190911182546.17094-1-nsaenzjulienne@suse.de/T/ Baoquan He (3): arm64: kdump : take off the protection on crashkernel memory region arm64: kdump: do not map crashkernel region specifically arm64: kdump: defer the crashkernel reservation for platforms with no DMA memory zones arch/arm64/include/asm/kexec.h | 6 ----- arch/arm64/include/asm/memory.h | 5 ---- arch/arm64/kernel/machine_kexec.c | 20 -------------- arch/arm64/mm/init.c | 34 +++--------------------- arch/arm64/mm/mmu.c | 43 ------------------------------- 5 files changed, 3 insertions(+), 105 deletions(-)