linux-kernel - Re: [PATCH 1/2] mm/zsmalloc: don't hold locks of all pages when free

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Message-ID: <20240228060113.GJ11972@google.com>
Date: Wed, 28 Feb 2024 15:01:13 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: yosryahmed@...gle.com, Sergey Senozhatsky <senozhatsky@...omium.org>,
	hannes@...xchg.org, nphamcs@...il.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm/zsmalloc: don't hold locks of all pages when
 free_zspage()

On (24/02/27 03:02), Chengming Zhou wrote:
>  static void __free_zspage(struct zs_pool *pool, struct size_class *class,
>  				struct zspage *zspage)
>  {
> @@ -834,13 +841,12 @@ static void __free_zspage(struct zs_pool *pool, struct size_class *class,
>  		VM_BUG_ON_PAGE(!PageLocked(page), page);

Who owns page lock here if free_zspage() doesn't trylock_zspage() no longer?

>  		next = get_next_page(page);
>  		reset_page(page);
> -		unlock_page(page);
>  		dec_zone_page_state(page, NR_ZSPAGES);
>  		put_page(page);
>  		page = next;
>  	} while (page != NULL);
>  
> -	cache_free_zspage(pool, zspage);
> +	call_rcu(&zspage->rcu_head, rcu_free_zspage);
>  
>  	class_stat_dec(class, ZS_OBJS_ALLOCATED, class->objs_per_zspage);
>  	atomic_long_sub(class->pages_per_zspage, &pool->pages_allocated);
> @@ -852,16 +858,6 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
>  	VM_BUG_ON(get_zspage_inuse(zspage));
>  	VM_BUG_ON(list_empty(&zspage->list));
>  
> -	/*
> -	 * Since zs_free couldn't be sleepable, this function cannot call
> -	 * lock_page. The page locks trylock_zspage got will be released
> -	 * by __free_zspage.
> -	 */
> -	if (!trylock_zspage(zspage)) {
> -		kick_deferred_free(pool);
> -		return;
> -	}
> -
>  	remove_zspage(class, zspage);
>  	__free_zspage(pool, class, zspage);
>  }