linux-kernel - Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized memmaps in memory

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Date:   Mon, 21 Oct 2019 11:44:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()

On 19.10.19 04:05, Andrew Morton wrote:
> On Thu, 10 Oct 2019 09:17:42 +0200 David Hildenbrand <david@...hat.com> wrote:
> 
>>>> -	pgmap = get_dev_pagemap(pfn, NULL);
>>>> -	if (pgmap)
>>>> -		return memory_failure_dev_pagemap(pfn, flags, pgmap);
>>>> -
>>>> -	p = 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 != 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.
> 
> 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). */

?

-- 

Thanks,

David / dhildenb