On Mon, Mar 06, 2023 at 04:41:24PM +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 crashkernel high region crossing low and high memory boudary will bring > issues: > > 1) For crashkernel=x,high, if getting crashkernel high region across > low and high memory boundary, then user will see two memory regions in > low memory, and one memory region in high memory. The two crashkernel > low memory regions are confusing as shown in above example. > > 2) If people explicityly specify "crashkernel=x,high crashkernel=y,low" > and y <= 128M, when crashkernel high region crosses low and high memory > boundary and the part of crashkernel high reservation below boundary 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. > > 3) The crossing boundary behaviour of crahskernel high reservation is > different than x86 arch. On x86_64, the low memory end is 4G fixedly, > and the memory near 4G is reserved by system, e.g for mapping firmware, > pci mapping, so the crashkernel reservation crossing boundary never happens. > >From distros point of view, this brings inconsistency and confusion. Users > need to dig into x86 and arm64 system details to find out why. > > For kernel itself, the impact of issue 3) could be slight. While issue > 1) and 2) cause actual impact because it brings obscure semantics and > behaviour to crashkernel=,high reservation. > > Here, for crashkernel=xM,high, search the high memory for the suitable > region only in high memory. If failed, try reserving the suitable > region only in low memory. Like this, the crashkernel high region will > only exist in high memory, and crashkernel low region only exists in low > memory. The reservation behaviour for crashkernel=,high is clearer and > simpler. > > Note: On arm64, the high and low memory boudary could be 1G if it's RPi4 > system, or 4G if other normal systems. > > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx> Reviewed-by: Simon Horman <horms@xxxxxxxxxx> _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
