Currently, reservation of crashkernel low memory sometimes fails due to a sparse memory caused by physical kaslr with the following message: Cannot reserve 256MB crashkernel low memory, please try smaller size. Kdump needs low memory, memory area less than 4GB, e.g. for swiotlb. Its size is 256MB for low memory by default. OTOH, physical kaslr loads kernel images in a random physical address for security. Physical kaslr sometimes choose low memory and sparse there and as a result, reservation of crash kernel low memory could fail. This failure seldom occurs on systems with large memory. For example, on our system with 128GB, the issue occurs once in hundreds of reboots. Although it doesn't occur frequently and can be avoided in practice simply by rebooting the system, it definitely occurs once in hundreds of reboots. Once the issue occurs, it's difficult for ordinary users to understand why it failed. I'd like to fix this current behavior. I'm now coming up two ideas but don't know what is best. Please discuss how to fix the issue in better way. 1) Add a kernel parameter to make physical kaslr to avoid specified memory area This is the simplest idea I came up with first just like kaslr_mem_avoid=4GB-0, which is similar syntax to memap=, meaning that kaslr, please avoid to load kernel image into the region [0, 4GB). It looks to me that this can be implemented easily by taking advantage of the existing code about mem_avoid mechanism in arch/x86/boot/compressed/kaslr.c. This mechanism doesn't lose security provided by physical kaslr if system memory is large enough. Demerit of this is that users need configuration. Automatic way is better if possible. 2) Add special handling for crashkernel= low in physical kaslr The second idea I came up with is to add special handling for crashkernel= low in physical kaslr, i.e. physical kaslr recognizes crashkernel= in kernel parameters and keep enough memory for crashkenrel. To guarantee that the memory area kept by the special handling in physical kaslr is used for crashkernel, it is necessary to mark the area to indicate to the crashkernel code executed after kernel runs. To implement this, I imagine introducing a new type of memory a kind of E820_CRASHKERNEL_LOW. My concern on this idea is whether its worth implementing such special handling in physical kaslr simply because I don't find such code in physical kaslr now. 3) Any other better ideas? Thanks. HATAYAMA, Daisuke _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec