linux-kernel - Re: [PATCH] mm: swap: prejudgement swap_has

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Message-ID: <CAKEwX=O8_arvHJy7ZUZP+j4zY8Yj=Z4iGEgDh4B-GG0cnHphLw@mail.gmail.com>
Date: Mon, 1 Apr 2024 08:20:40 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: Zhaoyu Liu <liuzhaoyu.zackary@...edance.com>, akpm@...ux-foundation.org, 
	ying.huang@...el.com, songmuchun@...edance.com, david@...hat.com, 
	willy@...radead.org, chrisl@...nel.org, yosryahmed@...gle.com, 
	guo.ziliang@....com.cn, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: swap: prejudgement swap_has_cache to avoid page allocation

On Mon, Apr 1, 2024 at 8:15 AM Kairui Song <ryncsn@...il.com> wrote:
> eg. If swap_has_cache returns true, then swap cache is freed, and
> skip_if_exists is set to true, __read_swap_cache_async will return
> NULL for an entry that it should be able to alloc and cache, could
> this be a problem (for example, causing zswap writeback to fail with
> ENOMEM due to readahead)?

Yeah the scenario you describe sounds plausible to me. This will make
zswap writeback less efficient - it seems more pages will be
(unnecessarily) skipped because of this check.