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Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()
To:     Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.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>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <134d4f03-a40a-fe62-fb93-53d209a91d2e@redhat.com>
Date:   Thu, 10 Oct 2019 09:17:42 +0200
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On 10.10.19 02:26, Naoya Horiguchi wrote:
> On Wed, Oct 09, 2019 at 04:24:35PM +0200, David Hildenbrand wrote:
>> We should check for pfn_to_online_page() to not access uninitialized
>> memmaps. Reshuffle the code so we don't have to duplicate the error
>> message.
>>
>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  mm/memory-failure.c | 14 ++++++++------
>>  1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 7ef849da8278..e866e6e5660b 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1253,17 +1253,19 @@ int memory_failure(unsigned long pfn, int flags)
>>  	if (!sysctl_memory_failure_recovery)
>>  		panic("Memory failure on page %lx", pfn);
>>  
>> -	if (!pfn_valid(pfn)) {
>> +	p = pfn_to_online_page(pfn);
>> +	if (!p) {
>> +		if (pfn_valid(pfn)) {
>> +			pgmap = get_dev_pagemap(pfn, NULL);
>> +			if (pgmap)
>> +				return memory_failure_dev_pagemap(pfn, flags,
>> +								  pgmap);
>> +		}
>>  		pr_err("Memory failure: %#lx: memory outside kernel control\n",
>>  			pfn);
>>  		return -ENXIO;
>>  	}
>>  
>> -	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.

-- 

Thanks,

David / dhildenb