On 28.08.25 17:10, Lorenzo Stoakes wrote:

On Thu, Aug 28, 2025 at 12:01:15AM +0200, David Hildenbrand wrote:

quoted

Let's limit the maximum folio size in problematic kernel config where
the memmap is allocated per memory section (SPARSEMEM without
SPARSEMEM_VMEMMAP) to a single memory section.

Currently, only a single architectures supports ARCH_HAS_GIGANTIC_PAGE
but not SPARSEMEM_VMEMMAP: sh.

Fortunately, the biggest hugetlb size sh supports is 64 MiB
(HUGETLB_PAGE_SIZE_64MB) and the section size is at least 64 MiB
(SECTION_SIZE_BITS == 26), so their use case is not degraded.

As folios and memory sections are naturally aligned to their order-2 size
in memory, consequently a single folio can no longer span multiple memory
sections on these problematic kernel configs.

nth_page() is no longer required when operating within a single compound
page / folio.

Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: David Hildenbrand <redacted>

Realy great comments, like this!

I wonder if we could have this be part of the first patch where you fiddle
with MAX_FOLIO_ORDER etc. but not a big deal.

I think it belongs into this patch where we actually impose the 
restrictions.

[...]

quoted

+/*
+ * Only pages within a single memory section are guaranteed to be
+ * contiguous. By limiting folios to a single memory section, all folio
+ * pages are guaranteed to be contiguous.
+ */
+#define MAX_FOLIO_ORDER		PFN_SECTION_SHIFT

Hmmm, was this implicit before somehow? I mean surely by the fact as you say
that physical contiguity would not otherwise be guaranteed :))

Well, my patches until this point made sure that any attempt to use a 
larger folio would fail in a way that we could spot now if there is any 
offender.

That is why before this change, nth_page() was required within a folio.

Hope that clarifies it, thanks!

-- 
Cheers

David / dhildenb