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. Extra words =========== I personally expect that we can add these back in the near future when arm64_dma_phys_limit is fixed, e.g Raspberry Pi enlarges the device addressing limit to 32bit; or Arm64 can support splitting built block or section mapping. Like this, the code is the simplest and clearest. Or as Catalin suggested, for below 4 cases we currently defer to handle in bootme_init(), we can try to handle case 3) in advance so that memory above 4G can avoid base page mapping wholly. This will complicate the already complex code, let's see how it looks if people interested post patch. crashkernel=size 1)first attempt: low memory under arm64_dma_phys_limit 2)fallback: finding memory above 4G crashkernel=size,high 3)first attempt: finding memory above 4G 4)fallback: low memory under arm64_dma_phys_limit [1] https://lore.kernel.org/all/YrIIJkhKWSuAqkCx@xxxxxxx/T/#u [2] https://lore.kernel.org/linux-arm-kernel/20190911182546.17094-1-nsaenzjulienne@xxxxxxx/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 | 6 +---- arch/arm64/mm/mmu.c | 43 ------------------------------- 5 files changed, 1 insertion(+), 79 deletions(-) -- 2.34.1 _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
