On 06/07/19 at 07:30pm, Borislav Petkov wrote: > On Mon, Jan 28, 2019 at 11:18:31AM +0100, Borislav Petkov wrote: > > On Mon, Jan 28, 2019 at 05:58:09PM +0800, Dave Young wrote: > > > Another reason is in case ,high we will need automatically reserve a > > > region in low area for swiotlb. So for example one use > > > crashkernel=256M,high, actual reserved memory is 256M above 4G and > > > another 256M under 4G for swiotlb. Normally it is not necessary for > > > most people. Thus we can not make ,high as default. > > > > And how is the poor user to figure out that we decided for her/him that > > swiotlb reservation is something not necessary for most people and thus > > we fail the crashkernel= reservation? > > > > IOW, that "logic" above doesn't make a whole lot of sense to me from > > user friendliness perspective. > > So to show what I mean: I'm trying to reserve a crash kernel region on a > box here. I tried: > > crashkernel=64M@16M > > as it is stated in Documentation/kdump/kdump.txt. > > Box said: > > [ 0.000000] crashkernel reservation failed - memory is in use. > > Oh great. > > Then I tried: > > crashkernel=64M@64M > > Box said: > > [ 0.000000] crashkernel reservation failed - memory is in use. > > So I simply did: > > crashkernel=64M > > and the box said: > > [ 0.000000] Reserving 64MB of memory at 3392MB for crashkernel (System RAM: 16271MB) > > So I could've gone a long time poking at the memory to find a suitable > address. > > So do you see what I mean with making this as user-friendly and as > robust as possible? Yes, it is clear to me, I absolutely agree that is not friendly :) Previously without KASLR, one can check /proc/iomem to find a possible free area and use it for next and future boot. But in case KASLR enabled nowadays it become harder to predict the persistent free areas. > > In this case I don't care about *where* my crash kernel is - I only want > to have one loaded *somewhere*. We would suggest people to use crashkernel=X instead. for the X@Y I believe it is some historic thing, it *should* be able to be obsolete at least on X86, (not sure other arches). I expect people can comment if they have some use cases requiring this X@Y way. We have modified the crashkernel=X to search 0 - 4G memory instead of old 0 - 896M for low memory areas, so a possible case is people who uses very old kexec-tools which can only load kernel to memory under 896M. Another way is we just obsolete X@Y, but introduce another interface like crahskernel=X,max= (max will be used like the CRASH_ADDR_HIGH_MAX in arch/x86/kernel/setup.c) > > And the same strategy should be applied to other reservation attempts > - we should try hard to reserve and if we cannot reserve, then try an > alternating range. > > I even think that > > crashkernel=X@Y > > should not simply fail if Y is occupied but keep trying and say > > [ 0.000000] Reserving 64MB of memory at alternative address 3392MB for crashkernel (System RAM: 16271MB) > > and only fail when the user doesn't really want the kernel to try hard > by booting with > > crashkernel=X@Y,strict > > But that's for another day. Maybe X@Y,max=.. Then kernel will search begin with Y, and stop until max - 1; > > -- > Regards/Gruss, > Boris. > > Good mailing practices for 400: avoid top-posting and trim the reply. Thanks Dave _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
