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

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Date: Fri, 29 Mar 2024 13:44:57 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,
	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: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages_enabled

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").

Thanks for prodiving context.