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Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Michal Hocko <mhocko@kernel.org>
References: <20191009142435.3975-1-david@redhat.com>
 <20191009142435.3975-3-david@redhat.com>
 <20191010002619.GB3585@hori.linux.bs1.fc.nec.co.jp>
 <134d4f03-a40a-fe62-fb93-53d209a91d2e@redhat.com>
 <20191018190531.975b70fabdce5f7e5d6b27df@linux-foundation.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <15e0a869-4edf-a5c4-64b1-5ed25a6ff720@redhat.com>
Date:   Mon, 21 Oct 2019 11:44:19 +0200
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On 19.10.19 04:05, Andrew Morton wrote:
> On Thu, 10 Oct 2019 09:17:42 +0200 David Hildenbrand <david@redhat.com> w=
rote:
>=20
>>>> -=09pgmap =3D get_dev_pagemap(pfn, NULL);
>>>> -=09if (pgmap)
>>>> -=09=09return memory_failure_dev_pagemap(pfn, flags, pgmap);
>>>> -
>>>> -=09p =3D pfn_to_page(pfn);
>>>
>>> This change seems to assume that memory_failure_dev_pagemap() is never
>>> called for online pages. Is it an intended behavior?
>>> Or the concept "online pages" is not applicable to zone device pages?
>>
>> Yes, that's the real culprit. ZONE_DEVICE/devmem pages are never online
>> (SECTION_IS_ONLINE). The terminology "online" only applies to pages that
>> were given to the buddy. And as we support sup-section hotadd for
>> devmem, we cannot easily make use of the section flag it. I already
>> proposed somewhere to convert SECTION_IS_ONLINE to a subsection bitmap
>> and call it something like pfn_active().
>>
>> pfn_online() would then be "pfn_active() && zone !=3D ZONE_DEVICE". And =
we
>> could use pfn_active() everywhere to test for initialized memmaps (well,
>> besides some special cases like device reserved memory that does not
>> span full sub-sections). Until now, nobody volunteered and I have other
>> things to do.
>=20
> Is it worth a code comment or two to make this clearer?

You mean something like

/* Only pages managed by the buddy are online (not ZONE_DEVICE). */

?

--=20

Thanks,

David / dhildenb