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Date: Mon, 15 Apr 2024 21:15:25 +0200
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Subject: Re: [PATCH v2 2/5] mm: zswap: calculate limits only when updated
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
 Andrew Morton <akpm@linux-foundation.org>, Nhat Pham <nphamcs@gmail.com>,
 Chengming Zhou <chengming.zhou@linux.dev>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
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From: David Hildenbrand <david@redhat.com>
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On 15.04.24 20:30, Yosry Ahmed wrote:
> On Mon, Apr 15, 2024 at 8:10 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 13.04.24 03:05, Yosry Ahmed wrote:
>>> On Fri, Apr 12, 2024 at 12:48 PM David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> On 10.04.24 02:52, Yosry Ahmed wrote:
>>>>> [..]
>>>>>>> Do we need a separate notifier chain for totalram_pages() updates?
>>>>>>
>>>>>> Good question. I actually might have the requirement to notify some arch
>>>>>> code (s390x) from virtio-mem when fake adding/removing memory, and
>>>>>> already wondered how to best wire that up.
>>>>>>
>>>>>> Maybe we can squeeze that into the existing notifier chain, but needs a
>>>>>> bit of thought.
>>>>>
>>>>
>>>> Sorry for the late reply, I had to think about this a bit.
>>>>
>>>>> Do you mean by adding new actions (e.g. MEM_FAKE_ONLINE,
>>>>> MEM_FAKE_OFFLINE), or by reusing the existing actions (MEM_ONLINE,
>>>>> MEM_OFFLINE, etc).
>>>>
>>>> At least for virtio-mem, I think we could have a MEM_ONLINE/MEM_OFFLINE
>>>> that prepare the whole range belonging to the Linux memory block
>>>> (/sys/devices/system/memory/memory...) to go online, and then have
>>>> something like MEM_SOFT_ONLINE/MEM_SOFT_OFFLINE or
>>>> ENABLE_PAGES/DISABLE_PAGES ... notifications when parts become usable
>>>> (!PageOffline, handed to the buddy) or unusable (PageOffline, removed
>>>> from the buddy).
>>>>
>>>> There are some details to be figured out, but it could work.
>>>>
>>>> And as virtio-mem currently operates in pageblock granularity (e.g., 2
>>>> MiB), but frequently handles multiple contiguous pageblocks within a
>>>> Linux memory block, it's not that bad.
>>>>
>>>>
>>>> But the issue I see with ballooning is that we operate here often on
>>>> page granularity. While we could optimize some cases, we might get quite
>>>> some overhead from all the notifications. Alternatively, we could send a
>>>> list of pages, but it won't win a beauty contest.
>>>>
>>>> I think the main issue is that, for my purpose (virtio-mem on s390x), I
>>>> need to notify about the exact memory ranges (so I can reinitialize
>>>> stuff in s390x code when memory gets effectively re-enabled). For other
>>>> cases (total pages changing), we don't need the memory ranges, but only
>>>> the "summary" -- or a notification afterwards that the total pages were
>>>> just changed quite a bit.
>>>
>>>
>>> Thanks for shedding some light on this. Although I am not familiar
>>> with ballooning, sending notifications on page granularity updates
>>> sounds terrible. It seems like this is not as straightforward as I had
>>> anticipated.
>>>
>>> I was going to take a stab at this, but given that the motivation is a
>>> minor optimization on the zswap side, I will probably just give up :)
>>
>> Oh no, so I have to do the work! ;)
>>
>>>
>>> For now, I will drop this optimization from the series for now, and I
>>> can revisit it if/when notifications for totalram_pages() are
>>> implemented at some point. Please CC me if you do so for the s390x use
>>> case :)
>>
>> I primarily care about virtio-mem resizing VM memory and adjusting
>> totalram_pages(), memory ballooning for that is rather a hack for that
>> use case ... so we're in agreement :)
>>
>> Likely we'd want two notification mechanisms, but no matter how I look
>> at it, it's all a bit ugly.
> 
> I am assuming you mean one with exact memory ranges for your s390x use
> case, and one high-level mechanism for totalram_pages() updates -- or
> did I miss the point?

No, that's it.

> 
> I am interested to see how page granularity updates would be handled
> in this case. Perhaps they are only relevant for the high-level
> mechanism? In that case, I suppose we can batch updates and notify
> once when a threshold is crossed or something.

Yes, we'd batch updates.

-- 
Cheers,

David / dhildenb