Re: [PATCH v2 2/3] mm/memory_hotplug: Introduce MHP_NO_FIRMWARE_MEMMAP

[David Hildenbrand <david@xxxxxxxxxx>]

On 30.04.20 18:33, Eric W. Biederman wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
> 
>> On 30.04.20 17:38, Eric W. Biederman wrote:
>>> David Hildenbrand <david@xxxxxxxxxx> writes:
>>>
>>>> Some devices/drivers that add memory via add_memory() and friends (e.g.,
>>>> dax/kmem, but also virtio-mem in the future) don't want to create entries
>>>> in /sys/firmware/memmap/ - primarily to hinder kexec from adding this
>>>> memory to the boot memmap of the kexec kernel.
>>>>
>>>> In fact, such memory is never exposed via the firmware memmap as System
>>>> RAM (e.g., e820), so exposing this memory via /sys/firmware/memmap/ is
>>>> wrong:
>>>>  "kexec needs the raw firmware-provided memory map to setup the
>>>>   parameter segment of the kernel that should be booted with
>>>>   kexec. Also, the raw memory map is useful for debugging. For
>>>>   that reason, /sys/firmware/memmap is an interface that provides
>>>>   the raw memory map to userspace." [1]
>>>>
>>>> We don't have to worry about firmware_map_remove() on the removal path.
>>>> If there is no entry, it will simply return with -EINVAL.
>>>>
>>>> [1]
>>>> https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-firmware-memmap
>>>
>>>
>>> You know what this justification is rubbish, and I have previously
>>> explained why it is rubbish.
>>
>> Actually, no, I don't think it is rubbish. See patch #3 and the cover
>> letter why this is the right thing to do *for special memory*, *not
>> ordinary DIMMs*.
>>
>> And to be quite honest, I think your response is a little harsh. I don't
>> recall you replying to my virtio-mem-related comments.
>>
>>>
>>> Nacked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>>>
>>> This needs to be based on weather the added memory is ultimately normal
>>> ram or is something special.
>>
>> Yes, that's what the caller are expected to decide, see patch #3.
>>
>> kexec should try to be as closely as possible to a real reboot - IMHO.
> 
> That is very fuzzy in terms of hotplug memory.  The kexec'd kernel
> should see the hotplugged memory assuming it is ordinary memory.
> 
> But kexec is not a reboot although it is quite similar.   Kexec is
> swapping one running kernel and it's state for another kernel without
> rebooting.

I agree (especially regarding the arm64 DIMM hotplug discussion).
However, for the two cases

a) dax/kmem
b) virtio-mem

We really want to let the driver take back control and figure out "what
to do with the memory".

> 
>>> Justifying behavior by documentation that does not consider memory
>>> hotplug is bad thinking.
>>
>> Are you maybe confusing this patch series with the arm64 approach? This
>> is not about ordinary hotplugged DIMMs.
> 
> I think I am.
> 
> My challenge is that I don't see anything in the description that says
> this isn't about ordinary hotplugged DIMMs.  All I saw was hotplug
> memory.

I'm sorry if that was confusing, I tried to stress that kmem and
virtio-mem is special in the description.

I squeezed a lot of that information into the cover letter and into
patch #3.

> 
> If the class of memory is different then please by all means let's mark
> it differently in struct resource so everyone knows it is different.
> But that difference needs to be more than hotplug.
> 
> That difference needs to be the hypervisor loaned us memory and might
> take it back at any time, or this memory is persistent and so it has
> these different characteristics so don't use it as ordinary ram.

Yes, and I think kmem took an excellent approach of explicitly putting
that "System RAM" into a resource hierarchy. That "System RAM" won't
show up as a root node under /proc/iomem (see patch #3), which already
results in kexec-tools to treat it in a special way. I am thinking about
doing the same for virtio-mem.

> 
> That information is also useful to other people looking at the system
> and seeing what is going on.
> 
> Just please don't muddle the concepts, or assume that whatever subset of
> hotplug memory you are dealing with is the only subset.

I can certainly rephrase the subject/description/comment, stating that
this is not to be used for ordinary hotplugged DIMMs - only when the
device driver is under control to decide what to do with that memory -
especially when kexec'ing.

(previously, I called this flag MHP_DRIVER_MANAGED, but I think
MHP_NO_FIRMWARE_MEMMAP is clearer, we just need a better description)

Would that make it clearer?

Thanks!

-- 
Thanks,

David / dhildenb