arm64: account for sparsemem section alignment when choosing vmemmap offset

Commit dfd55ad8 ("arm64: vmemmap: use virtual projection of linear
region") fixed an issue where the struct page array would overflow into the
adjacent virtual memory region if system RAM was placed so high up in
physical memory that its addresses were not representable in the build time
configured virtual address size.

However, the fix failed to take into account that the vmemmap region needs
to be relatively aligned with respect to the sparsemem section size, so that
a sequence of page structs corresponding with a sparsemem section in the
linear region appears naturally aligned in the vmemmap region.

So round up vmemmap to sparsemem section size. Since this essentially moves
the projection of the linear region up in memory, also revert the reduction
of the size of the vmemmap region.

Cc: <stable@vger.kernel.org>
Fixes: dfd55ad8 ("arm64: vmemmap: use virtual projection of linear region")
Tested-by: Mark Langsdorf <mlangsdo@redhat.com>
Tested-by: David Daney <david.daney@cavium.com>
Tested-by: Robert Richter <rrichter@cavium.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -40,7 +40,7 @@
  * VMALLOC_END: extends to the available space below vmmemmap, PCI I/O space,
  *	fixed mappings and modules
  */
-#define VMEMMAP_SIZE		ALIGN((1UL << (VA_BITS - PAGE_SHIFT - 1)) * sizeof(struct page), PUD_SIZE)
+#define VMEMMAP_SIZE		ALIGN((1UL << (VA_BITS - PAGE_SHIFT)) * sizeof(struct page), PUD_SIZE)
 
 #ifndef CONFIG_KASAN
 #define VMALLOC_START		(VA_START)
@@ -52,7 +52,8 @@
 #define VMALLOC_END		(PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
 
 #define VMEMMAP_START		(VMALLOC_END + SZ_64K)
-#define vmemmap			((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
+#define vmemmap			((struct page *)VMEMMAP_START - \
+				 SECTION_ALIGN_DOWN(memstart_addr >> PAGE_SHIFT))
 
 #define FIRST_USER_ADDRESS	0UL