On Fri, Aug 29, 2025 at 01:57:22PM +0200, David Hildenbrand wrote:
On 28.08.25 17:10, Lorenzo Stoakes wrote:
On Thu, Aug 28, 2025 at 12:01:15AM +0200, David Hildenbrand wrote:
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 david@redhat.com
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.
Sure it's not a big deal.
[...]
+/*
- 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.
Ack yeah.
That is why before this change, nth_page() was required within a folio.
Hope that clarifies it, thanks!
Yes thanks! :)
-- Cheers
David / dhildenb
Cheers, Lorenzo