1 ======================================= 2 crashkernel memory reservation on arm64 3 ======================================= 4 5 Author: Baoquan He <bhe@redhat.com> 6 7 Kdump mechanism is used to capture a corrupted kernel vmcore so that 8 it can be subsequently analyzed. In order to do this, a preliminarily 9 reserved memory is needed to pre-load the kdump kernel and boot such 10 kernel if corruption happens. 11 12 That reserved memory for kdump is adapted to be able to minimally 13 accommodate the kdump kernel and the user space programs needed for the 14 vmcore collection. 15 16 Kernel parameter 17 ================ 18 19 Through the kernel parameters below, memory can be reserved accordingly 20 during the early stage of the first kernel booting so that a continuous 21 large chunk of memomy can be found. The low memory reservation needs to 22 be considered if the crashkernel is reserved from the high memory area. 23 24 - crashkernel=size@offset 25 - crashkernel=size 26 - crashkernel=size,high crashkernel=size,low 27 28 Low memory and high memory 29 ========================== 30 31 For kdump reservations, low memory is the memory area under a specific 32 limit, usually decided by the accessible address bits of the DMA-capable 33 devices needed by the kdump kernel to run. Those devices not related to 34 vmcore dumping can be ignored. On arm64, the low memory upper bound is 35 not fixed: it is 1G on the RPi4 platform but 4G on most other systems. 36 On special kernels built with CONFIG_ZONE_(DMA|DMA32) disabled, the 37 whole system RAM is low memory. Outside of the low memory described 38 above, the rest of system RAM is considered high memory. 39 40 Implementation 41 ============== 42 43 1) crashkernel=size@offset 44 -------------------------- 45 46 The crashkernel memory must be reserved at the user-specified region or 47 fail if already occupied. 48 49 50 2) crashkernel=size 51 ------------------- 52 53 The crashkernel memory region will be reserved in any available position 54 according to the search order: 55 56 Firstly, the kernel searches the low memory area for an available region 57 with the specified size. 58 59 If searching for low memory fails, the kernel falls back to searching 60 the high memory area for an available region of the specified size. If 61 the reservation in high memory succeeds, a default size reservation in 62 the low memory will be done. Currently the default size is 128M, 63 sufficient for the low memory needs of the kdump kernel. 64 65 Note: crashkernel=size is the recommended option for crashkernel kernel 66 reservations. The user would not need to know the system memory layout 67 for a specific platform. 68 69 3) crashkernel=size,high crashkernel=size,low 70 --------------------------------------------- 71 72 crashkernel=size,(high|low) are an important supplement to 73 crashkernel=size. They allows the user to specify how much memory needs 74 to be allocated from the high memory and low memory respectively. On 75 many systems the low memory is precious and crashkernel reservations 76 from this area should be kept to a minimum. 77 78 To reserve memory for crashkernel=size,high, searching is first 79 attempted from the high memory region. If the reservation succeeds, the 80 low memory reservation will be done subsequently. 81 82 If reservation from the high memory failed, the kernel falls back to 83 searching the low memory with the specified size in crashkernel=,high. 84 If it succeeds, no further reservation for low memory is needed. 85 86 Notes: 87 88 - If crashkernel=,low is not specified, the default low memory 89 reservation will be done automatically. 90 91 - if crashkernel=0,low is specified, it means that the low memory 92 reservation is omitted intentionally.
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