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

[Michal Hocko <mhocko@suse.com>]

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?

-- 
Michal Hocko
SUSE Labs