linux-kernel - Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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Date:   Mon, 29 Jul 2019 10:06:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Rashmica Gupta <rashmica.g@...il.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...een.com,
        Jonathan.Cameron@...wei.com, anshuman.khandual@....com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

>> Of course, other interfaces might make sense.
>>
>> You can then start using these memory blocks and hinder them from
>> getting onlined (as a safety net) via memory notifiers.
>>
>> That would at least avoid you having to call
>> add_memory/remove_memory/offline_pages/device_online/modifying
>> memblock
>> states manually.
> 
> I see what you're saying and that definitely sounds safer.
> 
> We would still need to call remove_memory and add_memory from memtrace
> as
> just offlining memory doesn't remove it from the linear page tables
> (if 
> it's still in the page tables then hardware can prefetch it and if
> hardware tracing is using it then the box checkstops).

That prefetching part is interesting (and nasty as well). If we could at
least get rid of the manual onlining/offlining, I would be able to sleep
better at night ;) One step at a time.

> 
>>
>> (binding the memory block devices to a driver would be nicer, but the
>> infrastructure is not really there yet - we have no such drivers in
>> place yet)
>>
>>> I don't know the mm code nor how the notifiers work very well so I
>>> can't quite see how the above would work. I'm assuming memtrace
>>> would
>>> register a hotplug notifier and when memory is offlined from
>>> userspace,
>>> the callback func in memtrace would be called if the priority was
>>> high
>>> enough? But how do we know that the memory being offlined is
>>> intended
>>> for usto touch? Is there a way to offline memory from userspace not
>>> using sysfs or have I missed something in the sysfs interface?
>>
>> The notifier would really only be used to hinder onlining as a safety
>> net. User space prepares (offlines) the memory blocks and then tells
>> the
>> drivers which memory blocks to use.
>>
>>> On a second read, perhaps you are assuming that memtrace is used
>>> after
>>> adding new memory at runtime? If so, that is not the case. If not,
>>> then
>>> would you be able to clarify what I'm not seeing?
>>
>> The main problem I see is that you are calling
>> add_memory/remove_memory() on memory your device driver doesn't own.
>> It
>> could reside on a DIMM if I am not mistaking (or later on
>> paravirtualized memory devices like virtio-mem if I ever get to
>> implement them ;) ).
> 
> This is just for baremetal/powernv so shouldn't affect virtual memory
> devices.

Good to now.

> 
>>
>> How is it guaranteed that the memory you are allocating does not
>> reside
>> on a DIMM for example added via add_memory() by the ACPI driver?
> 
> Good point. We don't have ACPI on powernv but currently this would try
> to remove memory from any online memory node, not just the ones that
> are backed by RAM. oops.

Okay, so essentially no memory hotplug/unplug along with memtrace. (can
we document that somewhere?). I think add_memory()/try_remove_memory()
could be tolerable in these environments (as it's only boot memory).

-- 

Thanks,

David / dhildenb