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

From: Baolin Wang
Date: Wed Apr 19 2023 - 02:47:45 EST




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.