Messages in this thread | | | Date | Fri, 18 Oct 2019 10:48:25 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH V9 2/2] arm64/mm: Enable memory hot remove |
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On Fri, Oct 11, 2019 at 08:26:32AM +0530, Anshuman Khandual wrote: > On 10/10/2019 05:04 PM, Catalin Marinas wrote: > > Mark Rutland mentioned at some point that, as a preparatory patch to > > this series, we'd need to make sure we don't hot-remove memory already > > given to the kernel at boot. Any plans here? > > Hmm, this series just enables platform memory hot remove as required from > generic memory hotplug framework. The path here is triggered either from > remove_memory() or __remove_memory() which takes physical memory range > arguments like (nid, start, size) and do the needful. arch_remove_memory() > should never be required to test given memory range for anything including > being part of the boot memory.
Assuming arch_remove_memory() doesn't (cannot) check, is there a risk on arm64 that, for example, one removes memory available at boot and then kexecs a new kernel? Does the kexec tool present the new kernel with the original memory map?
I can see x86 has CONFIG_FIRMWARE_MEMMAP suggesting that it is used by kexec. try_remove_memory() calls firmware_map_remove() so maybe they solve this problem differently.
Correspondingly, after an arch_add_memory(), do we want a kexec kernel to access it? x86 seems to use the firmware_map_add_hotplug() mechanism.
Adding James as well for additional comments on kexec scenarios.
> IIUC boot memory added to system with memblock_add() lose all it's identity > after the system is up and running. In order to reject any attempt to hot > remove boot memory, platform needs to remember all those memory that came > early in the boot and then scan through it during arch_remove_memory(). > > Ideally, it is the responsibility of [_]remove_memory() callers like ACPI > driver, DAX etc to make sure they never attempt to hot remove a memory > range, which never got hot added by them in the first place. Also, unlike > /sys/devices/system/memory/probe there is no 'unprobe' interface where the > user can just trigger boot memory removal. Hence, unless there is a bug in > ACPI, DAX or other callers, there should never be any attempt to hot remove > boot memory in the first place.
That's fine if these callers give such guarantees. I just want to make sure someone checked all the possible scenarios for memory hot-remove.
-- Catalin
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