linux-kernel - Re: [External] Re: [PATCH] mm: zswap: fix the lack of page lru flag in zswap_writeback

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Message-ID: <CAJD7tkasktc68qtDE=9v6uZ-tvEyf-6X11B88bpisb+0x4gzHg@mail.gmail.com>
Date: Wed, 17 Jan 2024 09:57:44 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org, hannes@...xchg.org, 
	sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Chris Li <chrisl@...nel.org>, Michal Hocko <mhocko@...nel.org>
Subject: Re: [External] Re: [PATCH] mm: zswap: fix the lack of page lru flag
 in zswap_writeback_entry

On Wed, Jan 17, 2024 at 9:53 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> On Wed, Jan 17, 2024 at 1:53 AM Zhongkun He
> <hezhongkun.hzk@...edance.com> wrote:
> >
> > > >
> > > > Please forgive me for adding additional information about this patch.
> > > >
> > > > I have finished the opt for introducing a folio_add_lru_tail(), but
> > > > there are many
> > > > questions:
> > > > 1) A new page can be move to LRU only by lru_add_fn, so
> > > >     folio_add_lru_tail could not add pages to LRU for the following code
> > > >     in folio_batch_move_lru(),which is added by Alex Shi for
> > > >     serializing memcg changes in pagevec_lru_move_fn[1].
> > > >
> > > > /* block memcg migration while the folio moves between lru */
> > > >         if (move_fn != lru_add_fn && !folio_test_clear_lru(folio))
> > > >             continue;
> > > > To achieve the goal, we need to add a new function like  lru_add_fn
> > > > which does not have the lru flag and folio_add_lru_tail()
> > > > +               if (move_fn != lru_add_fn && move_fn != lru_move_tail_fn_new &&
> > > > +                       !folio_test_clear_lru(folio))
> > > >
> > > > 2)  __read_swap_cache_async has six parameters, so there is no space to
> > > > add a new one, add_to_lru_head.
> > > >
> > > > So it seems a bit hacky just for a special case for the reasons above.
> > >
> > > It's a lot of plumbing for sure. Adding a flag to current task_struct
> > > is a less-noisy yet-still-hacky solution. I am not saying we should do
> > > it, but it's an option. I am not sure how much task flags we have to
> > > spare.
> >
> > Got it.
>
> Actually this won't really work. Writebak can be asynchronous, so
> there would be no logical place to unset the flag.

Scratch that. We would only need the flag to be set until
folio_add_lru() is called (in __read_swap_cache_async()) -- so it can
work.

I am not sure if it's worth it though or if it's even acceptable. It
seems like current->flags are short of space.