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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP
On 01.05.20 20:43, Dan Williams wrote: > On Fri, May 1, 2020 at 11:14 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> On 01.05.20 20:03, Dan Williams wrote: >>> On Fri, May 1, 2020 at 10:51 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>> >>>> On 01.05.20 19:45, David Hildenbrand wrote: >>>>> On 01.05.20 19:39, Dan Williams wrote: >>>>>> On Fri, May 1, 2020 at 10:21 AM David Hildenbrand <david@xxxxxxxxxx> >>>>>> wrote: >>>>>>> >>>>>>> On 01.05.20 18:56, Dan Williams wrote: >>>>>>>> On Fri, May 1, 2020 at 2:34 AM David Hildenbrand <david@xxxxxxxxxx> >>>>>>>> wrote: >>>>>>>>> >>>>>>>>> On 01.05.20 00:24, Andrew Morton wrote: >>>>>>>>>> On Thu, 30 Apr 2020 20:43:39 +0200 David Hildenbrand >>>>>>>>>> <david@xxxxxxxxxx> wrote: >>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> Why does the firmware map support hotplug entries? >>>>>>>>>>> >>>>>>>>>>> I assume: >>>>>>>>>>> >>>>>>>>>>> The firmware memmap was added primarily for x86-64 kexec (and >>>>>>>>>>> still, is >>>>>>>>>>> mostly used on x86-64 only IIRC). There, we had ACPI hotplug. When >>>>>>>>>>> DIMMs >>>>>>>>>>> get hotplugged on real HW, they get added to e820. Same applies to >>>>>>>>>>> memory added via HyperV balloon (unless memory is unplugged via >>>>>>>>>>> ballooning and you reboot ... the the e820 is changed as well). I >>>>>>>>>>> assume >>>>>>>>>>> we wanted to be able to reflect that, to make kexec look like a >>>>>>>>>>> real reboot. >>>>>>>>>>> >>>>>>>>>>> This worked for a while. Then came dax/kmem. Now comes virtio-mem. >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> But I assume only Andrew can enlighten us. >>>>>>>>>>> >>>>>>>>>>> @Andrew, any guidance here? Should we really add all memory to the >>>>>>>>>>> firmware memmap, even if this contradicts with the existing >>>>>>>>>>> documentation? (especially, if the actual firmware memmap will *not* >>>>>>>>>>> contain that memory after a reboot) >>>>>>>>>> >>>>>>>>>> For some reason that patch is misattributed - it was authored by >>>>>>>>>> Shaohui Zheng <shaohui.zheng@xxxxxxxxx>, who hasn't been heard from >>>>>>>>>> in >>>>>>>>>> a decade. I looked through the email discussion from that time and >>>>>>>>>> I'm >>>>>>>>>> not seeing anything useful. But I wasn't able to locate Dave >>>>>>>>>> Hansen's >>>>>>>>>> review comments. >>>>>>>>> >>>>>>>>> Okay, thanks for checking. I think the documentation from 2008 is >>>>>>>>> pretty >>>>>>>>> clear what has to be done here. I will add some of these details to >>>>>>>>> the >>>>>>>>> patch description. >>>>>>>>> >>>>>>>>> Also, now that I know that esp. kexec-tools already don't consider >>>>>>>>> dax/kmem memory properly (memory will not get dumped via kdump) and >>>>>>>>> won't really suffer from a name change in /proc/iomem, I will go back >>>>>>>>> to >>>>>>>>> the MHP_DRIVER_MANAGED approach and >>>>>>>>> 1. Don't create firmware memmap entries >>>>>>>>> 2. Name the resource "System RAM (driver managed)" >>>>>>>>> 3. Flag the resource via something like IORESOURCE_MEM_DRIVER_MANAGED. >>>>>>>>> >>>>>>>>> This way, kernel users and user space can figure out that this memory >>>>>>>>> has different semantics and handle it accordingly - I think that was >>>>>>>>> what Eric was asking for. >>>>>>>>> >>>>>>>>> Of course, open for suggestions. >>>>>>>> >>>>>>>> I'm still more of a fan of this being communicated by "System RAM" >>>>>>> >>>>>>> I was mentioning somewhere in this thread that "System RAM" inside a >>>>>>> hierarchy (like dax/kmem) will already be basically ignored by >>>>>>> kexec-tools. So, placing it inside a hierarchy already makes it look >>>>>>> special already. >>>>>>> >>>>>>> But after all, as we have to change kexec-tools either way, we can >>>>>>> directly go ahead and flag it properly as special (in case there will >>>>>>> ever be other cases where we could no longer distinguish it). >>>>>>> >>>>>>>> being parented especially because that tells you something about how >>>>>>>> the memory is driver-managed and which mechanism might be in play. >>>>>>> >>>>>>> The could be communicated to some degree via the resource hierarchy. >>>>>>> >>>>>>> E.g., >>>>>>> >>>>>>> [root@localhost ~]# cat /proc/iomem >>>>>>> ... >>>>>>> 140000000-33fffffff : Persistent Memory >>>>>>> 140000000-1481fffff : namespace0.0 >>>>>>> 150000000-33fffffff : dax0.0 >>>>>>> 150000000-33fffffff : System RAM (driver managed) >>>>>>> >>>>>>> vs. >>>>>>> >>>>>>> :/# cat /proc/iomem >>>>>>> [...] >>>>>>> 140000000-333ffffff : virtio-mem (virtio0) >>>>>>> 140000000-147ffffff : System RAM (driver managed) >>>>>>> 148000000-14fffffff : System RAM (driver managed) >>>>>>> 150000000-157ffffff : System RAM (driver managed) >>>>>>> >>>>>>> Good enough for my taste. >>>>>>> >>>>>>>> What about adding an optional /sys/firmware/memmap/X/parent attribute. >>>>>>> >>>>>>> I really don't want any firmware memmap entries for something that is >>>>>>> not part of the firmware provided memmap. In addition, >>>>>>> /sys/firmware/memmap/ is still a fairly x86_64 specific thing. Only mips >>>>>>> and two arm configs enable it at all. >>>>>>> >>>>>>> So, IMHO, /sys/firmware/memmap/ is definitely not the way to go. >>>>>> >>>>>> I think that's a policy decision and policy decisions do not belong in >>>>>> the kernel. Give the tooling the opportunity to decide whether System >>>>>> RAM stays that way over a kexec. The parenthetical reference otherwise >>>>>> looks out of place to me in the /proc/iomem output. What makes it >>>>>> "driver managed" is how the kernel handles it, not how the kernel >>>>>> names it. >>>>> >>>>> At least, virtio-mem is different. It really *has to be handled* by the >>>>> driver. This is not a policy. It's how it works. >>> >>> ...but that's not necessarily how dax/kmem works. >>> >> >> Yes, and user space could still take that memory and add it to the >> firmware memmap if it really wants to. It knows that it is special. It >> can figure out that it belongs to a dax device using /proc/iomem. >> >>>>> >>>> >>>> Oh, and I don't see why "System RAM (driver managed)" would hinder any >>>> policy in user case to still do what it thinks is the right thing to do >>>> (e.g., for dax). >>>> >>>> "System RAM (driver managed)" would mean: Memory is not part of the raw >>>> firmware memmap. It was detected and added by a driver. Handle with >>>> care, this is special. >>> >>> Oh, no, I was more reacting to your, "don't update >>> /sys/firmware/memmap for the (driver managed) range" choice as being a >>> policy decision. It otherwise feels to me "System RAM (driver >>> managed)" adds confusion for casual users of /proc/iomem and for clued >>> in tools they have the parent association to decide policy. >> >> Not sure if I understand correctly, so bear with me :). >> >> Adding or not adding stuff to /sys/firmware/memmap is not a policy >> decision. If it's not part of the raw firmware-provided memmap, it has >> nothing to do in /sys/firmware/memmap. That's what the documentation >> from 2008 tells us. > > It just occurs to me that there are valid cases for both wanting to > start over with driver managed memory with a kexec and keeping it in > the map. Yes, there might be valid cases. My gut feeling is that in the general case, you want to let the kexec kernel implement a policy/ let the user in the new system decide. But as I said, you can implement in kexec-tools whatever policy you want. It has access to all information. > Consider the case of EFI Special Purpose (SP) Memory that is > marked EFI Conventional Memory with the SP attribute. In that case the > firmware memory map marked it as conventional RAM, but the kernel > optionally marks it as System RAM vs Soft Reserved. The 2008 patch > simply does not consider that case. I'm not sure strict textualism > works for coding decisions. I am no expert on that matter (esp EFI). But looking at the users of firmware_map_add_early(), the single user is in arch/x86/kernel/e820.c . So the single source of /sys/firmware/memmap is (besides hotplug) e820. "'e820_table_firmware': the original firmware version passed to us by the bootloader - not modified by the kernel. ... inform the user about the firmware's notion of memory layout via /sys/firmware/memmap" (arch/x86/kernel/e820.c) How is the EFI Special Purpose (SP) Memory represented in e820? /sys/firmware/memmap is really simple: just dump in e820. No policies IIUC. -- Thanks, David / dhildenb
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