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

[Baolin Wang]

On 4/14/2023 11:07 PM, Vlastimil Babka wrote:
> On 4/12/23 14:16, Baolin Wang wrote:
> > On 4/12/2023 7:25 PM, David Hildenbrand wrote:
> > > On 12.04.23 12:45, 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.
> > >
> > > pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.
> >
> > Yes, the granularity of memory hotplug is a mem_section.
> >
> > However, suppose the pageblock order is MAX_ORDER-1, and the size of a
> > sub-section is 2M, that means a pageblock will fall into 2 sub
> > mem-section, and if there is a hole in the zone, that means the 2nd sub
> > mem-section can be invalid without setting subsection_map bitmap.
>
> Can that really happen? I think the buddy merging in __free_one_page() would
> trip on that?

I do not think so IIUC. The hole pfns will not free to buddy system, and 
the buddy system did not change the ms->usage->subsection_map of the 
hole pfns, which indicates the hole pfns are invalid.