93 lines
3.4 KiB
ReStructuredText
93 lines
3.4 KiB
ReStructuredText
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=======================================
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crashkernel memory reservation on arm64
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=======================================
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Author: Baoquan He <bhe@redhat.com>
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Kdump mechanism is used to capture a corrupted kernel vmcore so that
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it can be subsequently analyzed. In order to do this, a preliminarily
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reserved memory is needed to pre-load the kdump kernel and boot such
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kernel if corruption happens.
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That reserved memory for kdump is adapted to be able to minimally
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accommodate the kdump kernel and the user space programs needed for the
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vmcore collection.
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Kernel parameter
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================
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Through the kernel parameters below, memory can be reserved accordingly
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during the early stage of the first kernel booting so that a continuous
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large chunk of memomy can be found. The low memory reservation needs to
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be considered if the crashkernel is reserved from the high memory area.
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- crashkernel=size@offset
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- crashkernel=size
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- crashkernel=size,high crashkernel=size,low
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Low memory and high memory
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==========================
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For kdump reservations, low memory is the memory area under a specific
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limit, usually decided by the accessible address bits of the DMA-capable
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devices needed by the kdump kernel to run. Those devices not related to
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vmcore dumping can be ignored. On arm64, the low memory upper bound is
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not fixed: it is 1G on the RPi4 platform but 4G on most other systems.
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On special kernels built with CONFIG_ZONE_(DMA|DMA32) disabled, the
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whole system RAM is low memory. Outside of the low memory described
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above, the rest of system RAM is considered high memory.
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Implementation
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==============
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1) crashkernel=size@offset
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--------------------------
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The crashkernel memory must be reserved at the user-specified region or
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fail if already occupied.
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2) crashkernel=size
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-------------------
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The crashkernel memory region will be reserved in any available position
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according to the search order:
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Firstly, the kernel searches the low memory area for an available region
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with the specified size.
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If searching for low memory fails, the kernel falls back to searching
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the high memory area for an available region of the specified size. If
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the reservation in high memory succeeds, a default size reservation in
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the low memory will be done. Currently the default size is 128M,
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sufficient for the low memory needs of the kdump kernel.
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Note: crashkernel=size is the recommended option for crashkernel kernel
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reservations. The user would not need to know the system memory layout
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for a specific platform.
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3) crashkernel=size,high crashkernel=size,low
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---------------------------------------------
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crashkernel=size,(high|low) are an important supplement to
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crashkernel=size. They allows the user to specify how much memory needs
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to be allocated from the high memory and low memory respectively. On
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many systems the low memory is precious and crashkernel reservations
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from this area should be kept to a minimum.
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To reserve memory for crashkernel=size,high, searching is first
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attempted from the high memory region. If the reservation succeeds, the
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low memory reservation will be done subsequently.
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If reservation from the high memory failed, the kernel falls back to
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searching the low memory with the specified size in crashkernel=,high.
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If it succeeds, no further reservation for low memory is needed.
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Notes:
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- If crashkernel=,low is not specified, the default low memory
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reservation will be done automatically.
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- if crashkernel=0,low is specified, it means that the low memory
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reservation is omitted intentionally.
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