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Message-ID: <19d5cdee-2868-41bd-83d5-6da75d72e940@maciej.szmigiero.name>
Date: Fri, 29 Mar 2024 15:02:10 +0100
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Subject: Re: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages_enabled
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To: Yosry Ahmed <yosryahmed@google.com>
Cc: 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, Johannes Weiner <hannes@cmpxchg.org>
References: <20240325235018.2028408-1-yosryahmed@google.com>
 <20240325235018.2028408-6-yosryahmed@google.com>
 <20240328191109.GE7597@cmpxchg.org>
 <CAJD7tka4iKD0=MOkj7iBCXvG6Jmq6WQgvRNV3mY+YRWPnYWmAg@mail.gmail.com>
 <CAJD7tkYo90ynKfBcWyMZLu7r-GJj3egnnJyGiJ5T2SN-Tn3d9A@mail.gmail.com>
From: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
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Disposition-Notification-To: "Maciej S. Szmigiero"
 <mail@maciej.szmigiero.name>
In-Reply-To: <CAJD7tkYo90ynKfBcWyMZLu7r-GJj3egnnJyGiJ5T2SN-Tn3d9A@mail.gmail.com>
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On 29.03.2024 03:14, Yosry Ahmed wrote:
> On Thu, Mar 28, 2024 at 1:06 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>>
>> On Thu, Mar 28, 2024 at 12:11 PM Johannes Weiner <hannes@cmpxchg.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@google.com>
>>>
>>> Acked-by: Johannes Weiner <hannes@cmpxchg.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