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Date: Tue, 20 Feb 2024 13:53:40 +0900
From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>, nphamcs@gmail.com,
	yosryahmed@google.com, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>, hannes@cmpxchg.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] mm/zsmalloc: remove migrate_write_lock_nested()
Message-ID: <20240220045340.GE11472@google.com>
References: <20240219-b4-szmalloc-migrate-v1-0-34cd49c6545b@bytedance.com>
 <20240219-b4-szmalloc-migrate-v1-2-34cd49c6545b@bytedance.com>
 <20240220044825.GD11472@google.com>
 <4f3c6c96-3bea-4369-a2de-e3c559505bba@bytedance.com>
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In-Reply-To: <4f3c6c96-3bea-4369-a2de-e3c559505bba@bytedance.com>

On (24/02/20 12:51), Chengming Zhou wrote:
> On 2024/2/20 12:48, Sergey Senozhatsky wrote:
> > On (24/02/19 13:33), Chengming Zhou wrote:
> >>  static void migrate_write_unlock(struct zspage *zspage)
> >>  {
> >>  	write_unlock(&zspage->lock);
> >> @@ -2003,19 +1997,17 @@ static unsigned long __zs_compact(struct zs_pool *pool,
> >>  			dst_zspage = isolate_dst_zspage(class);
> >>  			if (!dst_zspage)
> >>  				break;
> >> -			migrate_write_lock(dst_zspage);
> >>  		}
> >>  
> >>  		src_zspage = isolate_src_zspage(class);
> >>  		if (!src_zspage)
> >>  			break;
> >>  
> >> -		migrate_write_lock_nested(src_zspage);
> >> -
> >> +		migrate_write_lock(src_zspage);
> >>  		migrate_zspage(pool, src_zspage, dst_zspage);
> >> -		fg = putback_zspage(class, src_zspage);
> >>  		migrate_write_unlock(src_zspage);
> >>  
> >> +		fg = putback_zspage(class, src_zspage);
> > 
> > Hmm. Lockless putback doesn't look right to me. We modify critical
> > zspage fileds in putback_zspage().
> 
> Which I think is protected by pool->lock, right? We already held it.

Not really. We have, for example, the following patterns:

	get_zspage_mapping()
	spin_lock(&pool->lock)