arm64: kdump: Avoid reserving low memory repeatedly

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6Y5Y1



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We call reserve_crashkernel_high() before map_mem() to reserve high
memory in advance, which in turn can avoid using page level mapping for
all memory above 4G to optimize performance. And after
reserve_crashkernel_high(), reserve_crashkernel_low() is also needed to
reserve low memory. But when the system RAM is less than 4G, the memory
reserved by reserve_crashkernel_high() is already low memory (less than
4G), reserve_crashkernel_low() may reserve low memory again and the
memory it reserves may be higher than that reserved by
reserve_crashkernel_high(). Looking at /proc/iomem would have:

 # cat /proc/iomem | grep -i crash
    65400000-953fffff : Crash kernel  ==> crashk_res
    a7800000-b77fffff : Crash kernel  ==> crashk_res_low

At this point kexec-tools will incorrectly use the second memory segment
for the kdump kernel image load, causing the kernel load address check
to fail during kexec load (see sanity_check_segment_list()).

When the memory reserved by reserve_crashkernel_high() meets the low
memory requirement, reserve_crashkernel_low() is no longer called to
reserve memory and avoid introducing problems with duplicate
reservations.

Fixes: baac34dd ("arm64: kdump: Use page-level mapping for the high memory of crashkernel")
Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Reviewed-by: Yang Jihong <yangjihong1@huawei.com>