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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Rafael Aquini <aquini@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference
References: <20200922184838.978540-1-aquini@redhat.com>
        <20200922124750.67a20d9764ec098b17705407@linux-foundation.org>
        <20200923134251.GN795820@optiplex-lnx>
        <20200924195914.d240877a10e22b457bd9b45d@linux-foundation.org>
Date:   Fri, 25 Sep 2020 11:06:53 +0800
In-Reply-To: <20200924195914.d240877a10e22b457bd9b45d@linux-foundation.org>
        (Andrew Morton's message of "Thu, 24 Sep 2020 19:59:14 -0700")
Message-ID: <87v9g2blma.fsf@yhuang-dev.intel.com>
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Hi, Andrew,

Andrew Morton <akpm@linux-foundation.org> writes:

> On Wed, 23 Sep 2020 09:42:51 -0400 Rafael Aquini <aquini@redhat.com> wrote:
>
>> On Tue, Sep 22, 2020 at 12:47:50PM -0700, Andrew Morton wrote:
>> > 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?
>> >
>> 
>> UGH! I missed adding it to my cc list. Shall I just forward it, now, or
>> do you prefer a fresh repost?
>
> I added the cc:stable to my copy.

Please don't merge this patch.  This patch doesn't fix the bug, but hide
the real bug.  I will work with Rafael on root causing and fixing.

Best Regards,
Huang, Ying