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

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Date: Tue, 9 Jan 2024 11:13:40 +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>, weijie.yang@...sung.com
Subject: Re: [External] Re: [PATCH] mm: zswap: fix the lack of page lru flag
 in zswap_writeback_entry

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.

>
> 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.

 Thank you very much.