linux-kernel - Re: [PATCH 2/2 v2] mm/zsmalloc.c: Fix race condition in zs_destroy

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Message-ID: <20190820025939.GD500@jagdpanzerIV>
Date:   Tue, 20 Aug 2019 11:59:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Henry Burns <henryburns@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jonathan Adams <jwadams@...gle.com>,
        HenryBurns <henrywolfeburns@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2 v2] mm/zsmalloc.c: Fix race condition in
 zs_destroy_pool

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@...gle.com>

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

+ Andrew

	-ss