Re: [PATCH] mm/sparse: Fix kernel crash with pfn_section_valid check

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 3/25/20 1:07 PM, Baoquan He wrote:
> On 03/25/20 at 03:06pm, Baoquan He wrote:
>> On 03/25/20 at 08:49am, Aneesh Kumar K.V wrote:
> 
>>>   mm/sparse.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/mm/sparse.c b/mm/sparse.c
>>> index aadb7298dcef..3012d1f3771a 100644
>>> --- a/mm/sparse.c
>>> +++ b/mm/sparse.c
>>> @@ -781,6 +781,8 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
>>>   			ms->usage = NULL;
>>>   		}
>>>   		memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
>>> +		/* Mark the section invalid */
>>> +		ms->section_mem_map &= ~SECTION_HAS_MEM_MAP;
>>
>> Not sure if we should add checking in valid_section() or pfn_valid(),
>> e.g check ms->usage validation too. Otherwise, this fix looks good to
>> me.
> 
> With SPASEMEM_VMEMAP enabled, we should do validation check on ms->usage
> before checking any subsection is valid. Since now we do have case
> in which ms->usage is released, people still try to check it.
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f0a2c184eb9a..d79bd938852e 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1306,6 +1306,8 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
>   {
>   	int idx = subsection_map_index(pfn);
>   
> +	if (!ms->usage)
> +		return 0;
>   	return test_bit(idx, ms->usage->subsection_map);
>   }
>   #else
> 

We always check for section valid, before we check if pfn_section_valid().

static inline int pfn_valid(unsigned long pfn)

	struct mem_section *ms;

	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
		return 0;
	ms = __nr_to_section(pfn_to_section_nr(pfn));
	if (!valid_section(ms))
		return 0;
	/*
	 * Traditionally early sections always returned pfn_valid() for
	 * the entire section-sized span.
	 */
	return early_section(ms) || pfn_section_valid(ms, pfn);
}


IMHO adding that if (!ms->usage) is redundant.

-aneesh