Re: [PATCH v2 2/2] zswap: increment swapin count for non-pivot swapped in pages

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Aug 2, 2024 at 4:46 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Thu, Aug 1, 2024 at 1:02 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> >
> > Hmm, but there is a chance that these pages are not actually needed,
> > in which case we will unnecessarily increase the zswap protection.
> > Does the readahead window self-correct if the pages were not used?
>
> Hmm yeah it's kinda hard to predict if a swapped in page is strictly
> necessary in this context. We don't have this information at the time
> of the read.
>
> That said, I think erring on the side of safety is OK here - my
> understanding that readahead, while predictive in nature, only gets
> progressively more aggressive if we get signals that it's helpful (i.e
> the memory access patterns display sequential behavior).

If the readahead logic is expected to adapt in these situations (and I
think it is), then I think we are fine. Perhaps we should just leave a
comment that we may increase the protection more than we should for
those readahead cases.

>
> I think we also accept this slight inaccuracy (i.e for pages in the
> readahead window that might not necessarily be needed) the in
> workingset refault handling behavior. Could you fact check me,
> Johannes?
>
>
> >
> > > are also incrementing when the pages are read from the zswap pool, which
> > > is inaccurate.
> >
> > I feel like this is the more important part. It should be the focus of
> > the commit log with more details (i.e. why is it wrong to increment
> > the zswap protection in this case).
>
> Yeah this is pretty important too :) Maybe I should make it clearer in
> the patch commit.
>
> >
> > Do we need a Fixes and cc:stable for this one? Maybe it can be moved
> > first to make backports easy.
>
> Hmm.
>
> *Technically*, this is broken in older versions of the shrinker as
> well, but it's more of an optimization than a bug that can crash the
> kernel, so I don't know if it qualifies for a Fixes tag?
>
> Another factor is, under the old scheme, this does not move the needle
> much - at least in my benchmarks. This is because the decaying
> behavior is so aggressive that incrementing the counter in a couple
> places does not matter, when it will be rapidly divided by half later.
> This fix only shows clear improvements when applied on top of the new
> second chance scheme.
>
> I don't have a strong opinion here, but it doesn't seem worth it to
> backport IMHO :)

I thought it's a simple change worth backporting, but if it doesn't
move the needle without the second chance algorithm then it's probably
not worth it.

I would still add the "Fixes" tag because technically the logic is
wrong without this patch, it increases the zswap protection when there
swapins from zswap which doesn't make much sense.