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

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Date: Fri, 29 Mar 2024 15:02:10 +0100
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 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.

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.

On the other hand, "same_filled_pages_enabled" sounds like some kind
of a debugging option since I don't see any real reason to disable it
either.

Thanks,
Maciej