Re: [RFC PATCH 00/15] Make MAX_ORDER adjustable as a kernel boot time parameter.

From: David Hildenbrand
Date: Fri Aug 06 2021 - 12:16:48 EST

On 06.08.21 17:36, Vlastimil Babka wrote:
On 8/5/21 9:02 PM, Zi Yan wrote:
From: Zi Yan <ziy@xxxxxxxxxx>

Patch 3 restores the pfn_valid_within() check when buddy allocator can merge
pages across memory sections. The check was removed when ARM64 gets rid of holes
in zones, but holes can appear in zones again after this patchset.

To me that's most unwelcome resurrection. I kinda missed it was going away and
now I can't even rejoice? I assume the systems that will be bumping max_order
have a lot of memory. Are they going to have many holes? What if we just
sacrificed the memory that would have a hole and don't add it to buddy at all?

I think the old implementation was just horrible and the description we have here still suffers from that old crap: "but holes can appear in zones again". No, it's not related to holes in zones at all. We can have MAX_ORDER -1 pages that are partially a hole.

And to be precise, "hole" here means "there is no memmap" and not "there is a hole but it has a valid memmap".

But IIRC, we now have under SPARSEMEM always a complete memmap for a complete memory sections (when talking about system RAM, ZONE_DEVICE is different but we don't really care for now I think).

So instead of introducing what we had before, I think we should look into something that doesn't confuse each person that stumbles over it out there. What does pfn_valid_within() even mean in the new context? pfn_valid() is most probably no longer what we really want, as we're dealing with multiple sections that might be online or offline; in the old world, this was different, as a MAX_ORDER -1 page was completely contained in a memory section that was either online or offline.

I'd imagine something that expresses something different in the context of sparsemem:

"Some page orders, such as MAX_ORDER -1, might span multiple memory sections. Each memory section has a completely valid memmap if online. Memory sections might either be completely online or completely offline. pfn_to_online_page() might succeed on one part of a MAX_ORDER - 1 page, but not on another part. But it will certainly be consistent within one memory section."

Further, as we know that MAX_ORDER -1 and memory sections are a power of two, we can actually do a binary search to identify boundaries, instead of having to check each and every page in the range.

Is what I describe the actual reason why we introduce pfn_valid_within() ? (and might better introduce something new, with a better fitting name?)


David / dhildenb