Re: [PATCH v2 2/2] mm/page_alloc: add some comments to explain the possible hole in __pageblock_pfn_to_page()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 4/24/2023 5:54 PM, Michal Hocko wrote:
> On Sun 23-04-23 18:59:11, Baolin Wang wrote:
>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(), which
>> checks whether the given zone contains holes, and uses pfn_to_online_page()
>> to validate if the start pfn is online and valid, as well as using pfn_valid()
>> to validate the end pfn.
>>
>> However, the __pageblock_pfn_to_page() function may return non-NULL even
>> if the end pfn of a pageblock is in a memory hole in some situations. For
>> example, if the pageblock order is MAX_ORDER, which will fall into 2
>> sub-sections, and the end pfn of the pageblock may be hole even though
>> the start pfn is online and valid.
>>
>> This did not break anything until now, but the zone continuous is fragile
>> in this possible scenario. So as previous discussion[1], it is better to
>> add some comments to explain this possible issue in case there are some
>> future pfn walkers that rely on this.
>>
>> [1] https://lore.kernel.org/all/87r0sdsmr6.fsf@yhuang6-desk2.ccr.corp.intel.com/
> 
> Do I remember correctly you've had a specific configuration that would
> trigger this case?

Yes, I provided an example in previous thread [2] so show the 
__pageblock_pfn_to_page() is fragile in some cases.

[2] 
https://lore.kernel.org/all/52dfdd2e-9c99-eac4-233e-59919a24323e@linux.alibaba.com/

> 
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Changes from v1:
>>   - Update the comments per Ying and Mike, thanks.
>> ---
>>   mm/page_alloc.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 6457b64fe562..9756d66f471c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1502,6 +1502,13 @@ void __free_pages_core(struct page *page, unsigned int order)
>>    * interleaving within a single pageblock. It is therefore sufficient to check
>>    * the first and last page of a pageblock and avoid checking each individual
>>    * page in a pageblock.
>> + *
>> + * Note: the function may return non-NULL even if the end pfn of a pageblock
>> + * is in a memory hole in some situations. For example, if the pageblock
>> + * order is MAX_ORDER, which will fall into 2 sub-sections, and the end pfn
>> + * of the pageblock may be hole even though the start pfn is online and valid.
>> + * This did not break anything until now, but be careful about this possible
>> + * issue when checking whether all pfns of a pageblock are valid.
> 
> It is not really clear what you should be doing (other than to be
> careful which is not helpful much TBH) when you encounter this
> situation. If the reality changes and this would break in the future
> what would breakage look like? What should be done about that?

That depends on what the future pfn walkers do, which may access some 
hole memory with zero-init page frame. For example, if checking the 
__PageMovable() for a zero-init page frame, that will crash the system. 
But I can not list all the possible cases.

So how about below words?

  * Note: the function may return non-NULL even if the end pfn of a 
pageblock
  * is in a memory hole in some situations. For example, if the pageblock
  * order is MAX_ORDER, which will fall into 2 sub-sections, and the end pfn
  * of the pageblock may be hole even though the start pfn is online and 
valid.
  * This did not break anything until now, but be careful about this 
possible
  * issue when checking whether all pfns of a pageblock are valid, that may
  * lead to accessing empty page frame, and the worst case can crash the 
system.
  * So you should use pfn_to_onlie_page() instead of pfn_valid() to 
valid the
  * pfns in a pageblock if such case happens.