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

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Jan 8, 2024 at 7:13 PM Zhongkun He <hezhongkun.hzk@bytedance.com> wrote:
>
> Hi Yosry, glad to hear from you and happy new year!
>
> > Sorry for being late to the party. It seems to me that all of this
> > hassle can be avoided if lru_add_fn() did the right thing in this case
> > and added the folio to the tail of the lru directly. I am no expert in
> > how the page flags work here, but it seems like we can do something
> > like this in lru_add_fn():
> >
> > if (folio_test_reclaim(folio))
> >     lruvec_add_folio_tail(lruvec, folio);
> > else
> >     lruvec_add_folio(lruvec, folio);
> >
> > I think the main problem with this is that PG_reclaim is an alias to
> > PG_readahead, so readahead pages will also go to the tail of the lru,
> > which is probably not good.
>
> Agree with you， I will try it.

+Matthew Wilcox

I think we need to figure out if it's okay to do this first, because
it will affect pages with PG_readahead as well.

>
> >
> > A more intrusive alternative is to introduce a folio_lru_add_tail()
> > variant that always adds pages to the tail, and optionally call that
> > from __read_swap_cache_async() instead of folio_lru_add() based on a
> > new boolean argument. The zswap code can set that boolean argument
> > during writeback to make sure newly allocated folios are always added
> > to the tail of the lru.
>
> I have the same idea and also find it intrusive. I think the first solution
> is very good and I will try it. If it works, I will send the next version.

One way to avoid introducing folio_lru_add_tail() and blumping a
boolean from zswap is to have a per-task context (similar to
memalloc_nofs_save()), that makes folio_add_lru() automatically add
folios to the tail of the LRU. I am not sure if this is an acceptable
approach though in terms of per-task flags and such.