It doesn't make sense to have Kfence functionality in kdump kernel, hence disabling it in kdump kernel can reduce memory usage accordingly.

In the current fedora and RHEL, below config items related to kfence feature are set by default:
    
    ===
    CONFIG_HAVE_ARCH_KFENCE=y
    CONFIG_KFENCE=y
    CONFIG_KFENCE_SAMPLE_INTERVAL=100
    CONFIG_KFENCE_NUM_OBJECTS=255
    CONFIG_KFENCE_STRESS_TEST_FAULTS=0
    CONFIG_KFENCE_KUNIT_TEST=m
    ===
   

With them set, on x86_64, it will cost 2M extra memory used for kfence when page size if 4K; while on arm64 with 64K page size, it will cost 32M extra memory.