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

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Jan 10, 2024 at 7:49 PM Zhongkun He
<hezhongkun.hzk@bytedance.com> 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.
>
> >
> > 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.

I agree with Nhat here. Unless someone with enough readahead/page
flags knowledge says putting PG_readahead pages at the tail of the LRU
is okay (doubtful), I think we should opt for introducing a
folio_add_lru_tail() as I initially suggested.