On Thu, Apr 14, 2022 at 07:57:16PM +0800, Zhen Lei wrote: > /* > * reserve_crashkernel() - reserves memory for crash kernel > * > * This function reserves memory area given in "crashkernel=" kernel command > * line parameter. The memory reserved is used by dump capture kernel when > * primary kernel is crashing. > + * > + * NOTE: Reservation of crashkernel,low is special since its existence > + * is not independent, need rely on the existence of crashkernel,high. > + * Here, four cases of crashkernel low memory reservation are summarized: > + * 1) crashkernel=Y,low is specified explicitly, the size of crashkernel low > + * memory takes Y; > + * 2) crashkernel=,low is not given, while crashkernel=,high is specified, > + * take the default crashkernel low memory size; > + * 3) crashkernel=X is specified, while fallback to get a memory region > + * in high memory, take the default crashkernel low memory size; > + * 4) crashkernel='invalid value',low is specified, failed the whole > + * crashkernel reservation and bail out. Following the x86 behaviour made sense when we were tried to get that code generic. Now that we moved the logic under arch/arm64, we can diverge a bit. I lost track of the original (v1/v2) proposal but I wonder whether we still need the fallback to high for crashkernel=Y. Maybe simpler, no fallbacks: crashkernel=Y - keep the current behaviour, ignore high,low crashkernel=Y,high - allocate above ZONE_DMA crashkernel=Y,low - allocate within ZONE_DMA >From your proposal, the difference is that the Y,high option won't have any default ZONE_DMA fallback, one would have to explicitly pass the Y,low option if needed. Just a thought, maybe it makes the code simpler. But I'm open to discussion if there are good arguments for the proposed (x86-like) behaviour. One argument could be for crashkernel=Y to fall back to high if distros don't want to bother with high/low settings. Another thing I may have asked in the past, what happens if we run a new kernel with these patches with old kexec user tools. I suspect the crashkernel=Y with the fallback to high will confuse the tools. BTW, please separate the NO_BLOCK_MAPPINGS optimisations from the crashkernel above 4G. Let's get the crashkernel reservations sorted first, it's been around for too long. Thanks. -- Catalin
