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Date:   Tue, 22 Sep 2020 12:47:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Rafael Aquini <aquini@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        ying.huang@intel.com
Subject: Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer
 dereference
Message-Id: <20200922124750.67a20d9764ec098b17705407@linux-foundation.org>
In-Reply-To: <20200922184838.978540-1-aquini@redhat.com>
References: <20200922184838.978540-1-aquini@redhat.com>
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On Tue, 22 Sep 2020 14:48:38 -0400 Rafael Aquini <aquini@redhat.com> wrote:

> The swap area descriptor only gets struct swap_cluster_info *cluster_info
> allocated if the swapfile is backed by non-rotational storage.
> When the swap area is laid on top of ordinary disk spindles, lock_cluster()
> will naturally return NULL.
> 
> CONFIG_THP_SWAP exposes cluster_info infrastructure to a broader number of
> use cases, and split_swap_cluster(), which is the counterpart of split_huge_page()
> for the THPs in the swapcache, misses checking the return of lock_cluster before
> operating on the cluster_info pointer.
> 
> This patch addresses that issue by adding a proper check for the pointer
> not being NULL in the wrappers cluster_{is,clear}_huge(), in order to avoid
> crashes similar to the one below:
> 
> ...
>
> Fixes: 59807685a7e77 ("mm, THP, swap: support splitting THP for THP swap out")
> Signed-off-by: Rafael Aquini <aquini@redhat.com>

Did you consider cc:stable?