mm: move vma_kernel_pagesize() from hugetlb to mm.h

mm: move vma_kernel_pagesize() from hugetlb to mm.h

Patch series "mm: move vma_(kernel|mmu)_pagesize() out of hugetlb.c", v2.

Looking into vma_(kernel|mmu)_pagesize(), I realized that there is one
scenario where DAX would not do the right thing when the kernel is not
compiled with hugetlb support.

Without hugetlb support, vma_(kernel|mmu)_pagesize() will always return
PAGE_SIZE instead of using the ->pagesize() result provided by dax-device
code.

Fix that by moving vma_kernel_pagesize() to core MM code, where it
belongs.  I don't think this is stable material, but am not 100% sure.

Also, move vma_mmu_pagesize() while at it.  Remove the unnecessary
hugetlb.h inclusion from KVM code.

This patch (of 4):

In the past, only hugetlb had special "vma_kernel_pagesize()"
requirements, so it provided its own implementation.

In commit 05ea88608d4e ("mm, hugetlbfs: introduce ->pagesize() to
vm_operations_struct") we generalized that approach by providing a
vm_ops->pagesize() callback to be used by device-dax.

Once device-dax started using that callback in commit c1d53b92b95c
("device-dax: implement ->pagesize() for smaps to report MMUPageSize") it
was missed that CONFIG_DEV_DAX does not depend on hugetlb support.

So building a kernel with CONFIG_DEV_DAX but without CONFIG_HUGETLBFS
would not pick up that value.

Fix it by moving vma_kernel_pagesize() to mm.h, providing only a single
implementation.  While at it, improve the kerneldoc a bit.

Ideally, we'd move vma_mmu_pagesize() as well to the header.  However, its
__weak symbol might be overwritten by a PPC variant in hugetlb code.  So
let's leave it in there for now, as it really only matters for some
hugetlb oddities.

This was found by code inspection.

Link: https://lkml.kernel.org/r/20260309151901.123947-1-david@kernel.org
Link: https://lkml.kernel.org/r/20260309151901.123947-2-david@kernel.org
Fixes: c1d53b92b95c ("device-dax: implement ->pagesize() for smaps to report MMUPageSize")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>