Re: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages_enabled

[Maciej S. Szmigiero]

On 29.03.2024 19:22, Yosry Ahmed wrote:
> On Fri, Mar 29, 2024 at 10:45 AM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > > On Thu, Mar 28, 2024 at 12:11 PM Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxx>
> > > > > >
> > > > > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > > > >
> > > > > > 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@xxxxxxxxxx/
> > > >
> > > > 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