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Date:   Sun, 17 Jan 2021 14:01:42 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Naoya Horiguchi <nao.horiguchi@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>, stable@vger.kernel.org,
        linux-mm@kvack.org, linux-nvdimm@lists.01.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 4/5] mm: Fix page reference leak in
 soft_offline_page()
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On Wed, 13 Jan 2021 16:43:32 -0800 Dan Williams <dan.j.williams@intel.com> wrote:

> The conversion to move pfn_to_online_page() internal to
> soft_offline_page() missed that the get_user_pages() reference taken by
> the madvise() path needs to be dropped when pfn_to_online_page() fails.
> Note the direct sysfs-path to soft_offline_page() does not perform a
> get_user_pages() lookup.
> 
> When soft_offline_page() is handed a pfn_valid() &&
> !pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
> a leaked reference.
> 
> Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

A cc:stable patch in the middle is awkward.  I'll make this a
standalone patch for merging into mainline soon (for 5.11) and shall
turn the rest into a 4-patch series, OK?