linux-kernel - Re: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages

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Message-ID: <d111a52c-8f42-4743-9b1a-691882ca58be@maciej.szmigiero.name>
Date: Mon, 1 Apr 2024 12:37:56 +0200
From: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: 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, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages_enabled

On 29.03.2024 19:22, Yosry Ahmed wrote:
> On Fri, Mar 29, 2024 at 10:45 AM Johannes Weiner <hannes@...xchg.org> wrote:
>>
>> On Fri, Mar 29, 2024 at 03:02:10PM +0100, Maciej S. Szmigiero wrote:
>>> On 29.03.2024 03:14, Yosry Ahmed wrote:
>>>> On Thu, Mar 28, 2024 at 1:06 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>>>>>
>>>>> On Thu, Mar 28, 2024 at 12:11 PM Johannes Weiner <hannes@...xchg.org> wrote:
>>>>>>
>>>>>> On Mon, Mar 25, 2024 at 11:50:13PM +0000, Yosry Ahmed wrote:
>>>>>>> There is no logical reason to refuse storing same-filled pages more
>>>>>>> efficiently and opt for compression. Remove the userspace knob.
>>>>>>>
>>>>>>> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
>>>>>>
>>>>>> Acked-by: Johannes Weiner <hannes@...xchg.org>
>>>>>>
>>>>>> I also think the non_same_filled_pages_enabled option should go
>>>>>> away. Both of these tunables are pretty bizarre.
>>>>>
>>>>> Happy to remove both in the next version :)
>>>>
>>>> I thought non_same_filled_pages_enabled was introduced with the
>>>> initial support for same-filled pages, but it was introduced
>>>> separately (and much more recently):
>>>> https://lore.kernel.org/all/7dbafa963e8bab43608189abbe2067f4b9287831.1641247624.git.maciej.szmigiero@oracle.com/
>>>>
>>>> I am CCing Maciej to hear more about the use case for this.
>>>
>>> Thanks for CCing me.
>>>
>>> I introduced "non_same_filled_pages_enabled" a few years ago to
>>> enable using zswap in a lightweight mode where it is only used for
>>> its ability to store same-filled pages effectively.
>>
>> But all the pages it rejects go to disk swap instead, which is much
>> slower than compression...
>>
>>> As far as I remember, there were some interactions between full
>>> zswap and the cgroup memory controller - like, it made it easier
>>> for an aggressive workload to exceed its cgroup memory.high limits.
>>
>> Ok, that makes sense! A container fairness measure, rather than a
>> performance optimization.
>>
>> Fair enough, but that's moot then with cgroup accounting of the
>> backing memory, f4840ccfca25 ("zswap: memcg accounting").
> 
> Right, this should no longer be needed with the zswap charging.
> 
> Maciej, is this still being used on kernels with f4840ccfca25 (5.19+)?
> Any objections to removing it now?

I don't object to its removal as long as stable kernel trees aren't
affected.

Thanks,
Maciej