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According to our discussion about https://gitlab.com/cki-project/kernel-ark/-/merge_requests/1023
kexec-tools needs to disable CMA for kdump kernel cmdline. Otherwise kdump kernel may run out of memory.
For example one set crashkernel=1G, but if cma=512M is used, then kdump kernel will only have 512M. so we should strip the inherited cma= cmd line from 1st kernel grub. And may need to explictly set cma=0 since David said s390/ppc has some internal logic to set CMA.
------------original bug for kernel enabling cma on x86--------------
This bug was initially created as a copy of Bug #1945002
I am copying this bug because:
We want to enable/support CMA in RHEL9 on x86-64 (and eventually aarch64). Enabling CMA mostly involves enabling CONFIG_CMA and adding RHEL-specific warnings that CMA areas were defined / CMA allocations happened.
As one example CMA will be required to eventually support RDMA<->GPU p2pdma via dma-buf. As another example, CMA will be useful useful for more reliable runtime allocation of gigantic pages.
CMA is already enabled in ARK for s390x and ppc64le. For aarch64, 64k base page size currently implies a minimum CMA area size of 512 MiB (due to large pageblock order), which could be problematic on smaller machines.
Comment 2David Hildenbrand
2021-04-19 09:00:33 UTC
IIRC, to build the kdump ("crashkernel") cmdline we are not using the cmdline of the original kernel (I did a quick experiment and it looks like that is the behavior indeed); we only base our cmdline on the original cmline in case of ordinary kexec.
However, on ppc64, it might make sense to specify "kvm_cma_resv_ratio=0" for the kdump kernel, because the default in the kernel is set to "5%".