Re: [PATCH 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 4/23/2023 1:19 PM, Mike Rapoport wrote:
> Hi,
> 
> On Sat, Apr 22, 2023 at 06:15:18PM +0800, 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, though the start pfn of a pageblock is valid, it can not always
>> guarantee the end pfn of the pageblock is also valid (may be holes) in some
>> cases. For example, if the pageblock order is MAX_ORDER - 1, which will fall
> 
> Nit: in the current mm tree the default pageblock order is MAX_ORDER.

Ah, yes, will change in next version.

> 
>> 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/
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/page_alloc.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 6457b64fe562..dc4005b32ae0 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1502,6 +1502,14 @@ 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: if the start pfn of a pageblock is valid, but it can not always guarantee
>> + * the end pfn of the pageblock is also valid (may be holes) in some cases. For
>> + * example, if the pageblock order is MAX_ORDER - 1, 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 this possible issue when checking if the whole pfns are valid of a
> 
> careful about ...

OK. Thanks for reviewing.