Hi Dave/James, On 2020/1/17 11:58, Dave Young wrote: > On 01/16/20 at 03:17pm, James Morse wrote: >> Hi guys, >> >> On 28/12/2019 09:32, Dave Young wrote: >>> On 12/27/19 at 07:04pm, Chen Zhou wrote: >>>> On 2019/12/27 13:54, Dave Young wrote: >>>>> On 12/23/19 at 11:23pm, Chen Zhou wrote: >>>>>> In preparation for supporting reserve_crashkernel_low in arm64 as >>>>>> x86_64 does, move reserve_crashkernel_low() into kernel/crash_core.c. >>>>>> >>>>>> Note, in arm64, we reserve low memory if and only if crashkernel=X,low >>>>>> is specified. Different with x86_64, don't set low memory automatically. >>>>> >>>>> Do you have any reason for the difference? I'd expect we have same >>>>> logic if possible and remove some of the ifdefs. >>>> >>>> In x86_64, if we reserve crashkernel above 4G, then we call reserve_crashkernel_low() >>>> to reserve low memory. >>>> >>>> In arm64, to simplify, we call reserve_crashkernel_low() at the beginning of reserve_crashkernel() >>>> and then relax the arm64_dma32_phys_limit if reserve_crashkernel_low() allocated something. >>>> In this case, if reserve crashkernel below 4G there will be 256M low memory set automatically >>>> and this needs extra considerations. >> >>> Sorry that I did not read the old thread details and thought that is >>> arch dependent. But rethink about that, it would be better that we can >>> have same semantic about crashkernel parameters across arches. If we >>> make them different then it causes confusion, especially for >>> distributions. >> >> Surely distros also want one crashkernel* string they can use on all platforms without >> having to detect the kernel version, platform or changeable memory layout... >> >> >>> OTOH, I thought if we reserve high memory then the low memory should be >>> needed. There might be some exceptions, but I do not know the exact >>> one, >> >>> can we make the behavior same, and special case those systems which >>> do not need low memory reservation. >> >> Its tricky to work out which systems are the 'normal' ones. >> >> We don't have a fixed memory layout for arm64. Some systems have no memory below 4G. >> Others have no memory above 4G. >> >> Chen Zhou's machine has some memory below 4G, but its too precious to reserve a large >> chunk for kdump. Without any memory below 4G some of the drivers won't work. >> >> I don't see what distros can set as their default for all platforms if high/low are >> mutually exclusive with the 'crashkernel=' in use today. How did x86 navigate this, ... or >> was it so long ago? > > It is very rare for such machine without any low memory in X86, at least > from what I know, so the current way just works fine. > > Since arm64 is quite different, I would agree with current way > proposed in the patch, but a question is, for those arm64 systems how can > admin know if low crashkernel memory is needed or not? and just skip the > low reservation for machine with high memory installed only? Specified size low memory is for crash dump kernel devices. I think admin should know if there are devices needing low memory in crash dump kernel. James, any suggestions? Thanks, Chen Zhou > >> >> No one else has reported a problem with the existing placement logic, hence treating this >> 'low' thing as the 'in addition' special case. >> >> >>>> previous discusses: >>>> https://lkml.org/lkml/2019/6/5/670 >>>> https://lkml.org/lkml/2019/6/13/229 >>> >>> Another concern from James: >>> " >>> With both crashk_low_res and crashk_res, we end up with two entries in /proc/iomem called >>> "Crash kernel". Because its sorted by address, and kexec-tools stops searching when it >>> find "Crash kernel", you are always going to get the kernel placed in the lower portion. >>> " >>> >>> The kexec-tools code is iterating all "Crash kernel" ranges and add them >>> in an array. In X86 code, it uses the higher range to locate memory. >> >> Then my hurried reading of what the user-space code does was wrong! >> >> If kexec-tools places the kernel in the low region, there may not be enough memory left >> for whatever purpose it was reserved for. This was the motivation for giving it a >> different name. > > Agreed, it is still a potential problem though. Say we have both low > and high reserved. Kdump kernel boots up, the kernel and drivers, > applications will use memory, I'm not sure if there is a memory > allocation policy to let them all use high mem first.. Anyway that is > beyond the kexec-tools and resource name. > > Thanks > Dave > > > . > _______________________________________________ kexec mailing list kexec@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/kexec
