Re: [External] Re: [PATCH] mm: zswap: fix the lack of page lru flag in zswap_writeback_entry

[Zhongkun He]

On Fri, Jan 12, 2024 at 3:25 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> On Wed, Jan 10, 2024 at 7:49 PM Zhongkun He
> <hezhongkun.hzk@xxxxxxxxxxxxx> wrote:
> >
> > >
> > > This sounds dangerous. This is going to introduce a rather large
> > > unexpected side effect - we're changing the readahead behavior in a
> > > seemingly small zswap optimization. In fact, I'd argue that if we do
> > > this, the readahead behavior change will be the "main effect", and the
> > > zswap-side change would be a "happy consequence". We should run a lot
> > > of benchmarking and document the change extensively if we pursue this
> > > route.
> > >
> >
> > I agree with the unexpected side effect,  and here I need
> > to clarify the original intention of this patch.Please see the memory
> > offloading steps below.
> >
> >
> > memory      zswap(reclaim)          memory+swap (writeback)
> > 1G                 0.5G                        1G(tmp memory) + 1G（swap）
> >
> > If the decompressed memory cannot be released in time,
> > zswap's writeback has great side effects(mostly clod pages).
> > On the one hand, the memory space has not been reduced,
> > but has increased (from 0.5G->1G).
> > At the same time, it is not put the pages to the tail of the lru.
> > When the memory is insufficient, other pages will be squeezed out
> > and released early.
> > With this patch， we can put the tmp pages to the tail and reclaim it
> > in time when the memory is insufficient or actively reclaimed.
> > So I think this patch makes sense and hope it can be fixed with a
> > suitable approaches.
>
> Makes sense to me. IIUC, that's the original intention behind calling
> SetPageReclaim() - unfortunately that doesn't work :) And IIRC, your
> original attempt shows reduction in swap usage (albeit at the cost of
> performance regression), which means we're onto something. I believe
> that the folio_lru_add_tail() approach will work :)
>
> Please include a version of the clarification paragraph above in your
> later version to explain the goal of the optimization, along with
> suitable benchmark numbers to show the effect (such as minimal change
> in performance, and reduction in some metrics). Maybe include the link
> to the original patch that introduces SetPageReclaim() too, to show
> the motivation behind all of this :) It'd be nice to have all the
> contexts readily available, in case we need to revisit this in the
> future (as was the case with the SetPageReclaim() here).
>

OK.

> >
> > >
> > > Unless some page flag/readahead expert can confirm that the first
> > > option is safe, my vote is on this option. I mean, it's fairly minimal
> > > codewise, no? Just a bunch of plumbing. We can also keep the other
> > > call sites intact if we just rename the old versions - something along
> > > the line of:
> > >
> > > __read_swap_cache_async_head(..., bool add_to_lru_head)
> > > {
> > > ...
> > > if (add_to_lru_head)
> > >   folio_add_lru(folio)
> > > else
> > >   folio_add_lru_tail(folio);
> > > }
> > >
> > > __read_swap_cache_async(...)
> > > {
> > >    return __read_swap_cache_async_tail(..., true);
> > > }
> > >
> > > A bit boilerplate? Sure. But this seems safer, and I doubt it's *that*
> > > much more work.
> > >
> >
> > Yes， agree. I will try it again.
>
> Look forward to seeing it! Thanks for your patience and for working on this.

Thanks for your time.