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Date:   Tue, 20 Aug 2019 11:59:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Henry Burns <henryburns@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Minchan Kim <minchan@kernel.org>, Nitin Gupta <ngupta@vflare.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Shakeel Butt <shakeelb@google.com>,
        Jonathan Adams <jwadams@google.com>,
        HenryBurns <henrywolfeburns@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2 v2] mm/zsmalloc.c: Fix race condition in
 zs_destroy_pool
Message-ID: <20190820025939.GD500@jagdpanzerIV>
References: <20190809181751.219326-1-henryburns@google.com>
 <20190809181751.219326-2-henryburns@google.com>
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On (08/09/19 11:17), Henry Burns wrote:
> In zs_destroy_pool() we call flush_work(&pool->free_work). However, we
> have no guarantee that migration isn't happening in the background
> at that time.
> 
> Since migration can't directly free pages, it relies on free_work
> being scheduled to free the pages.  But there's nothing preventing an
> in-progress migrate from queuing the work *after*
> zs_unregister_migration() has called flush_work().  Which would mean
> pages still pointing at the inode when we free it.
> 
> Since we know at destroy time all objects should be free, no new
> migrations can come in (since zs_page_isolate() fails for fully-free
> zspages).  This means it is sufficient to track a "# isolated zspages"
> count by class, and have the destroy logic ensure all such pages have
> drained before proceeding.  Keeping that state under the class
> spinlock keeps the logic straightforward.
> 
> Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
> Signed-off-by: Henry Burns <henryburns@google.com>

Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

+ Andrew

	-ss