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

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Date: Tue, 16 Jan 2024 12:28:46 -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 Tue, Jan 16, 2024 at 5:40 AM Zhongkun He
<hezhongkun.hzk@...edance.com> wrote:
>
> > > > >
> > > > > 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.
>
> 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.

>
> 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.

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

[1]https://lore.kernel.org/linux-mm/ZaD9BNtXZfY2UtVI@tiehlicka/