On Fri, Feb 22, 2019 at 10:11:01AM +0800, Dave Young wrote: > In case people have a lot of devices need more swiotlb, then he manually > set the ,high with ,low together. The option to specify the high and low values for the crashkernel are important for certain machines. The point is that swiotlb already allocates 64MB of low memory by default. But that memory is only used for 32bit DMA-mask devices that want to DMA into high memory. There are drivers just allocating GFP_DMA32 memory, which also ends up in the low region (but not swiotlb), that is why the previous default of 72MB low memory was not enough, it only left 8MB of GFP_DMA32 memory. The current default of 256MB was found by experiments on a bigger number of machines, to create a reasonable default that is at least likely to be sufficient of an average machine. There is no way today for the kernel to find an optimum value for the amount of low memory required to successfully create a crash dump. It depends on the amount of devices in the system and how the drivers for them are written. The drivers have no way to report back their requirements, and even if they had, at the time the allocation happens no driver is loaded yet. So it is up to the system administrator to find workable values for the high and low memory requirements, even using experiments as a last resort. Regards, Joerg _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
