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Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
To:     Rashmica Gupta <rashmica.g@gmail.com>,
        Oscar Salvador <osalvador@suse.de>
Cc:     akpm@linux-foundation.org, mhocko@suse.com,
        dan.j.williams@intel.com, pasha.tatashin@soleen.com,
        Jonathan.Cameron@huawei.com, anshuman.khandual@arm.com,
        vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <0cd2c142-66ba-5b6d-bc9d-fe68c1c65c77@redhat.com>
Date:   Tue, 16 Jul 2019 14:28:01 +0200
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On 02.07.19 08:42, Rashmica Gupta wrote:
> Hi David,
> 
> Sorry for the late reply.

Hi,

sorry I was on PTO :)

> 
> On Wed, 2019-06-26 at 10:28 +0200, David Hildenbrand wrote:
>> On 26.06.19 10:15, Oscar Salvador wrote:
>>> On Wed, Jun 26, 2019 at 10:11:06AM +0200, David Hildenbrand wrote:
>>>> Back then, I already mentioned that we might have some users that
>>>> remove_memory() they never added in a granularity it wasn't
>>>> added. My
>>>> concerns back then were never fully sorted out.
>>>>
>>>> arch/powerpc/platforms/powernv/memtrace.c
>>>>
>>>> - Will remove memory in memory block size chunks it never added
>>>> - What if that memory resides on a DIMM added via
>>>> MHP_MEMMAP_DEVICE?
>>>>
>>>> Will it at least bail out? Or simply break?
>>>>
>>>> IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save
>>>> to be
>>>> introduced.
>>>
>>> Uhm, I will take a closer look and see if I can clear your
>>> concerns.
>>> TBH, I did not try to use arch/powerpc/platforms/powernv/memtrace.c
>>> yet.
>>>
>>> I will get back to you once I tried it out.
>>>
>>
>> BTW, I consider the code in arch/powerpc/platforms/powernv/memtrace.c
>> very ugly and dangerous.
> 
> Yes it would be nice to clean this up.
> 
>> We should never allow to manually
>> offline/online pages / hack into memory block states.
>>
>> What I would want to see here is rather:
>>
>> 1. User space offlines the blocks to be used
>> 2. memtrace installs a hotplug notifier and hinders the blocks it
>> wants
>> to use from getting onlined.
>> 3. memory is not added/removed/onlined/offlined in memtrace code.
>>
> 
> I remember looking into doing it a similar way. I can't recall the
> details but my issue was probably 'how does userspace indicate to
> the kernel that this memory being offlined should be removed'?

Instead of indicating a "size", indicate the offline memory blocks that
the driver should use. E.g. by memory block id for each node

0:20-24,1:30-32

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.

(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 ;) ).

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?

-- 

Thanks,

David / dhildenb