Re: [PATCH] mm/page_alloc: consider pfn holes after pfn_valid() in __pageblock_pfn_to_page()

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

On 4/12/2023 7:15 PM, Michal Hocko wrote:
> On Wed 12-04-23 18:45:31, 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_valid()
>> to check if the end pfn is valid. However pfn_valid() can not make sure
>> the end pfn is not a hole if the size of a pageblock is larger than the
>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>> may represent a hole or an unusable page frame, which may cause incorrect
>> zone contiguous is set.
>>
>> Though another user of pageblock_pfn_to_page() in compaction seems work
>> well now, it is better to avoid scanning or touching these offline pfns.
>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>> populated to have holes"), we should also use pfn_to_online_page() for
>> the end pfn to make sure it is a valid pfn with usable page frame.
>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>
>> Moreover we've already used pfn_to_online_page() for start pfn to make
>> sure it is online and valid, so the pfn_valid() for the start pfn is
>> unnecessary, drop it.
> 
> Is this a theoretical problem or something you have encountered on a
> real machine? Could you provide more details please?

As I replied to David, this is just from code inspection when trying to 
remove the unnecessary pfn_valid() in __pageblock_pfn_to_page().