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

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Message-ID: <CACSyD1PERLG_68OXwzuGH-cqOuS1k8N_fE5Xu-KACZ34UYephw@mail.gmail.com>
Date: Wed, 17 Jan 2024 17:52:47 +0800
From: Zhongkun He <hezhongkun.hzk@...edance.com>
To: Yosry Ahmed <yosryahmed@...gle.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

> >
> > 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.
>
> >
> > Back to the beginning,  lru_add_drain() is the simplest option，which is common
> > below the __read_swap_cache_async(). Please see the function
> > swap_cluster_readahead()
> > and swap_vma_readahead(), of course it has been batched.
> >
> > Or we should  leave this problem alone，before we can write back zswap
> > in batches.
>
> Calling lru_add_drain() for every written back page is an overkill
> imo. If we have writeback batching at some point, it may make more
> sense then.

Agree.

>
> Adding Michal Hocko was recently complaining [1] about lru_add_drain()
> being called unnecessarily elsewhere.

Got it, thanks.
>
> [1]https://lore.kernel.org/linux-mm/ZaD9BNtXZfY2UtVI@tiehlicka/