On Fri, Feb 03, 2023 at 03:57:23PM +0800, Baoquan He wrote: > On arm64, reservation for 'crashkernel=xM,high' is taken by searching for > suitable memory region top down. If the 'xM' of crashkernel high memory > is reserved from high memory successfully, it will try to reserve > crashkernel low memory later accoringly. Otherwise, it will try to search > low memory area for the 'xM' suitable region. Please see the details in > Documentation/admin-guide/kernel-parameters.txt. > > While we observed an unexpected case where a reserved region crosses the > high and low meomry boundary. E.g on a system with 4G as low memory end, > user added the kernel parameters like: 'crashkernel=512M,high', it could > finally have [4G-126M, 4G+386M], [1G, 1G+128M] regions in running kernel. > The crossing 4G boudary of crashkernel high region will bring issues: > > 1) For crashkernel=x,high, if getting crashkernel high region across > 4G boudary, then user will see two memory regions under 4G, and one > memory region above 4G. The two crashkernel low memory regions are > confusing. Looking at your patch, I just realised that the 4G boundary between 'low' and 'high' reservations is not always true. On RPi4, that would be 1GB, the limit of ZONE_DMA. Are there user-space tools that rely on this 32-bit boundary? If they do, they'd get confused on RPi4, not sure they have the notion of the actual ZONE_DMA that the kernel has enabled. If we do want ,high to mean always 4G or higher, we'd need to change the logic a bit so that the search_base starts from 4G rather than CRASH_ADDR_LOW_MAX. We could leave the latter when ,high was not specified. > 2) If people explicityly specify "crashkernel=x,high crashkernel=y,low" > and y <= 128M, when crashkernel high region crosses 4G boudary and the > part below 4G of crashkernel high reservation is bigger than y, the > expected crahskernel low reservation will be skipped. But the expected > crashkernel high reservation is shrank and could not satisfy user space > requirement. I guess if the user passes both high and low, we should honour that and ignore any y <= 128M checks. -- Catalin _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
