Date: Mon, 15 Apr 2024 17:10:48 +0200
From: David Hildenbrand <david@...hat.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>,
 Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] mm: zswap: calculate limits only when updated

On 13.04.24 03:05, Yosry Ahmed wrote:
> On Fri, Apr 12, 2024 at 12:48 PM David Hildenbrand <david@...hat.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'll look into the virtio-mem case soonish and will let you know once I 
have something running.

-- 
Cheers,

David / dhildenb

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