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

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Date:   Thu, 10 Oct 2019 09:27:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] mm/memory-failure.c: Don't access uninitialized
 memmaps in memory_failure()

On 09.10.19 16:43, Michal Hocko wrote:
> On Wed 09-10-19 16:24:35, 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@...jp.nec.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...nel.org>
>> Signed-off-by: David Hildenbrand <david@...hat.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;
> 
> Don't we need that earlier at hwpoison_inject level?
> 

Theoretically yes, this is another instance. But pfn_to_online_page(pfn)
alone would not be sufficient as discussed. We would, again, have to
special-case ZONE_DEVICE via things like get_dev_pagemap() ...

But mm/hwpoison-inject.c:hwpoison_inject() is a pure debug feature either way:

	/*
	 * Note that the below poison/unpoison interfaces do not involve
	 * hardware status change, hence do not require hardware support.
	 * They are mainly for testing hwpoison in software level.
	 */

So it's not that bad compared to memory_failure() called from real HW or
drivers/base/memory.c:soft_offline_page_store()/hard_offline_page_store()

-- 

Thanks,

David / dhildenb