linux-kernel - Re: [PATCH] mm, zswap: don't touch the XArray lock if there is no entry to free

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Message-ID: <ZxK7G3S0N42ejJMh@casper.infradead.org>
Date: Fri, 18 Oct 2024 20:46:35 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Kairui Song <kasong@...cent.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Chris Li <chrisl@...nel.org>, Barry Song <v-songbaohua@...o.com>,
	"Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, zswap: don't touch the XArray lock if there is no
 entry to free

On Sat, Oct 19, 2024 at 03:25:25AM +0800, Kairui Song wrote:
>  	if (xa_empty(tree))
>  		return;
>  
> -	entry = xa_erase(tree, offset);
> -	if (entry)
> +	rcu_read_lock();
> +	entry = xas_load(&xas);
> +	if (entry) {

You should call xas_reset() here.  And I'm not sure it's a great idea to
spin waiting for the xa lock while holding the RCU read lock?  Probably
not awful but I could easily be wrong.

> +		xas_lock(&xas);
> +		WARN_ON_ONCE(xas_reload(&xas) != entry);
> +		xas_store(&xas, NULL);
> +		xas_unlock(&xas);
>  		zswap_entry_free(entry);
> +	}
> +	rcu_read_unlock();
>  }
>  
>  int zswap_swapon(int type, unsigned long nr_pages)
> -- 
> 2.47.0
> 
>