Received-SPF: pass (google.com: domain of linux-kernel+bounces-145431-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <d4b5033a-34fd-44ed-ab57-ea2b4ed01468@redhat.com>
Date: Mon, 15 Apr 2024 17:10:48 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
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
References: <20240405053510.1948982-1-yosryahmed@google.com>
 <20240405053510.1948982-3-yosryahmed@google.com>
 <20240405152619.GA866431@cmpxchg.org>
 <CAJD7tkaSCvgs-yKXeAVoW14_LhtCrL3N9hu9xA41GCvz+RvWBw@mail.gmail.com>
 <7aec7b98-db81-4238-bdd6-afb69679f852@redhat.com>
 <CAJD7tkaa3P7dQys+LhuDe8kqWsUqf7PDB8Q+07+wnQ513-6NLg@mail.gmail.com>
 <69dcd33b-e8de-4927-93dd-d4ea22834a18@redhat.com>
 <CAJD7tkb5D6Y5OMTzpHefYgSXbw7GdPtfjSp2=nBGjJFk99GP_A@mail.gmail.com>
 <f18ecb84-31a5-4767-a8df-0c0b8be82d81@redhat.com>
 <CAJD7tkYMM9oDQYsWPcQNctRRH52+oM-3cUhxV+_7Qg1NNZx4cg@mail.gmail.com>
Content-Language: en-US
From: David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <CAJD7tkYMM9oDQYsWPcQNctRRH52+oM-3cUhxV+_7Qg1NNZx4cg@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

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'll look into the virtio-mem case soonish and will let you know once I 
have something running.

-- 
Cheers,

David / dhildenb