[PATCH v6 08/41] arm64: vmemmap: Avoid base2 order of struct page size to dimension region

[Mark Rutland]

On Wed, Nov 29, 2023 at 12:16:04PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> 
> The placement and size of the vmemmap region in the kernel virtual
> address space is currently derived from the base2 order of the size of a
> struct page. This makes for nicely aligned constants with lots of
> leading 0xf and trailing 0x0 digits, but given that the actual struct
> pages are indexed as an ordinary array, this resulting region is
> severely overdimensioned when the size of a struct page is just over a
> power of 2.
> 
> This doesn't matter today, but once we enable 52-bit virtual addressing
> for 4k pages configurations, the vmemmap region may take up almost half
> of the upper VA region with the current struct page upper bound at 64
> bytes. And once we enable KMSAN or other features that push the size of
> a struct page over 64 bytes, we will run out of VMALLOC space entirely.
> 
> So instead, let's derive the region size from the actual size of a
> struct page, and place the entire region 1 GB from the top of the VA
> space, where it still doesn't share any lower level translation table
> entries with the fixmap.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

This is nice, and largely addresses the fear I mentioned on the earlier patches
move the PCI_IO_* and FIXMAP_* regions. I'd still like to ensure that we have
assertions that those don't overlap with the VMEMMAP region, but assuming those
get added to the earlier patches, this looks good as-is:

Acked-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
>  arch/arm64/include/asm/memory.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 2745bed8ae5b..b49575a92afc 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -30,8 +30,8 @@
>   * keep a constant PAGE_OFFSET and "fallback" to using the higher end
>   * of the VMEMMAP where 52-bit support is not available in hardware.
>   */
> -#define VMEMMAP_SHIFT	(PAGE_SHIFT - STRUCT_PAGE_MAX_SHIFT)
> -#define VMEMMAP_SIZE	((_PAGE_END(VA_BITS_MIN) - PAGE_OFFSET) >> VMEMMAP_SHIFT)
> +#define VMEMMAP_RANGE	(_PAGE_END(VA_BITS_MIN) - PAGE_OFFSET)
> +#define VMEMMAP_SIZE	((VMEMMAP_RANGE >> PAGE_SHIFT) * sizeof(struct page))
>  
>  /*
>   * PAGE_OFFSET - the virtual address of the start of the linear map, at the
> @@ -47,8 +47,8 @@
>  #define MODULES_END		(MODULES_VADDR + MODULES_VSIZE)
>  #define MODULES_VADDR		(_PAGE_END(VA_BITS_MIN))
>  #define MODULES_VSIZE		(SZ_2G)
> -#define VMEMMAP_START		(-(UL(1) << (VA_BITS - VMEMMAP_SHIFT)))
> -#define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
> +#define VMEMMAP_START		(VMEMMAP_END - VMEMMAP_SIZE)
> +#define VMEMMAP_END		(-UL(SZ_1G))
>  #define PCI_IO_START		(VMEMMAP_END + SZ_8M)
>  #define PCI_IO_END		(PCI_IO_START + PCI_IO_SIZE)
>  #define FIXADDR_TOP		(-UL(SZ_8M))
> -- 
> 2.43.0.rc1.413.gea7ed67945-goog
>