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

Yosry Ahmed
2024-04-01T18:30:02Z

On Mon, Apr 1, 2024 at 3:38 AM Maciej S. Szmigiero
<mail@maciej.szmigiero.name> wrote:
>
> On 29.03.2024 19:22, Yosry Ahmed wrote:
> > On Fri, Mar 29, 2024 at 10:45 AM Johannes Weiner <hannes@cmpxchg.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@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.
> >>
> >> 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.

Yeah this isn't something that would be backported to stable kernels.
Thanks for confirming.