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Date:   Tue, 10 Apr 2018 08:48:44 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Minchan Kim <minchan@kernel.org>,
        Christopher Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, Jan Kara <jack@suse.cz>,
        Chris Fries <cfries@google.com>
Subject: Re: [PATCH] mm: workingset: fix NULL ptr dereference
Message-ID: <20180410124844.GC6334@cmpxchg.org>
References: <20180409015815.235943-1-minchan@kernel.org>
 <20180409024925.GA21889@bombadil.infradead.org>
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On Sun, Apr 08, 2018 at 07:49:25PM -0700, Matthew Wilcox wrote:
> @@ -2714,8 +2714,10 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  		stat(s, ALLOC_FASTPATH);
>  	}
>  
> -	if (unlikely(gfpflags & __GFP_ZERO) && object)
> -		memset(object, 0, s->object_size);
> +	if (unlikely(gfpflags & __GFP_ZERO) && object) {
> +		if (!WARN_ON_ONCE(s->ctor))
> +			memset(object, 0, s->object_size);
> +	}
>  
>  	slab_post_alloc_hook(s, gfpflags, 1, &object);

This looks like a useful check to have. But maybe behind DEBUG_VM?