On 2/25/21 7:05 PM, Mike Rapoport wrote:

On Thu, Feb 25, 2021 at 06:51:53PM +0100, Vlastimil Babka wrote:

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unset zone link in struct page will trigger

	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);

... in set_pfnblock_flags_mask() when called with a struct page from the
"Unknown E820 type" range.

"... in set_pfnblock_flags_mask() when called with a struct page from a range
other than E820_TYPE_RAM"

then :)

Better :)

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because there are pages in both ZONE_DMA32 and ZONE_DMA (unset zone link
in struct page) in the same pageblock.

I would say "there are apparently pages" ... "and ZONE_DMA does not span this range"

I'd rephrase it differently, something like

"because there are pages in the range of ZONE_DMA32 but the unset zone link
in struct page makes them appear as a part of ZONE_DMA"

Much better, thanks!

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Interleave initialization of the unavailable pages with the normal
initialization of memory map, so that zone and node information will be
properly set on struct pages that are not backed by the actual memory.

With this change the pages for holes inside a zone will get proper
zone/node links and the pages that are not spanned by any node will get
links to the adjacent zone/node.

What if two zones are adjacent? I.e. if the hole was at a boundary between two
zones.

What do you mean by "adjacent zones"? If there is a hole near the zone
boundary, zone span would be clamped to exclude the hole.

Yeah, zone span should exclude those pages, but you still somehow handle them?
That's how I read "pages that are not spanned by any node will get links to the
adjacent zone/node."
So is it always a unique zone/node can be determined?

Let's say we have:

<memory on node 0>
---- pageblock boundary ----
<more memory on node 0>
<a hole>
<memory on node 1>
---- pageblock boundary ----

Now I hope such configurations don't really exist :) But if we simulated them in
QEMU, what would be the linkage in struct pages in that hole?