arm64: kdump : take off the protection on crashkernel memory region

arm64: kdump : take off the protection on crashkernel memory region

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 base page mapping for the whole linear region, it's
better to take off the protection on crashkernel memory region for the
time being because the anticipated stamping on crashkernel memory region
could only happen in a chance in one million, while the base page mapping
for the whole linear region is mitigating arm64 systems with crashkernel
set always.

[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/

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20230407011507.17572-2-bhe@redhat.com
Signed-off-by: Will Deacon <will@kernel.org>