Re: [PATCH v3 05/60] arm64: mm: Move fixmap region above vmemmap region

Mark Rutland
2023-04-28T11:00:21Z

On Tue, Mar 07, 2023 at 03:04:27PM +0100, Ard Biesheuvel wrote:
> Move the fixmap region above the vmemmap region, so that the start of
> the vmemmap delineates the end of the region available for vmalloc and
> vmap allocations and the randomized placement of the kernel and modules.
>
> In a subsequent patch, we will take advantage of this to reclaim most of
> the vmemmap area when running a 52-bit VA capable build with 52-bit
> virtual addressing disabled at runtime.
>
> Note that the existing guard region of 256 MiB covers the fixmap and PCI
> I/O regions as well, so we can reduce it 8 MiB, which is what we use in
> other places too.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/arm64/include/asm/memory.h | 2 +-
> arch/arm64/include/asm/pgtable.h | 2 +-
> arch/arm64/mm/ptdump.c | 4 ++--
> 3 files changed, 4 insertions(+), 4 deletions(-)

As a heads-up, this will (trivially) conflict with some of the arm64 fixmap
cleanups that'll be in v6.4-rc1, due to the FIXADDR_TOT_* changes.

>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 6e321cc06a3c30f0..9b9e52d823beccc6 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -51,7 +51,7 @@
> #define VMEMMAP_END (VMEMMAP_START + VMEMMAP_SIZE)
> #define PCI_IO_START (VMEMMAP_END + SZ_8M)
> #define PCI_IO_END (PCI_IO_START + PCI_IO_SIZE)
> -#define FIXADDR_TOP (VMEMMAP_START - SZ_32M)
> +#define FIXADDR_TOP (ULONG_MAX - SZ_8M + 1)

Can we make this:

#define FIXADDR_TOP (-SZ_8M)

... as that would match the way we define PAGE_OFFSET (and VMEMMAP_START), and
it removes the need for the '+1' to handle ULONG_MAX being one-off what we
actually want to subtact from.

Mark.

>
> #if VA_BITS > 48
> #define VA_BITS_MIN (48)
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b6ba466e2e8a3fc7..3eff06c5d0eb73c7 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -22,7 +22,7 @@
> * and fixed mappings
> */
> #define VMALLOC_START (MODULES_END)
> -#define VMALLOC_END (VMEMMAP_START - SZ_256M)
> +#define VMALLOC_END (VMEMMAP_START - SZ_8M)
>
> #define vmemmap ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
>
> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> index 9d1f4cdc6672ed5f..76d28056bd14920a 100644
> --- a/arch/arm64/mm/ptdump.c
> +++ b/arch/arm64/mm/ptdump.c
> @@ -45,12 +45,12 @@ static struct addr_marker address_markers[] = {
> { MODULES_END, "Modules end" },
> { VMALLOC_START, "vmalloc() area" },
> { VMALLOC_END, "vmalloc() end" },
> - { FIXADDR_START, "Fixmap start" },
> - { FIXADDR_TOP, "Fixmap end" },
> { VMEMMAP_START, "vmemmap start" },
> { VMEMMAP_START + VMEMMAP_SIZE, "vmemmap end" },
> { PCI_IO_START, "PCI I/O start" },
> { PCI_IO_END, "PCI I/O end" },
> + { FIXADDR_START, "Fixmap start" },
> + { FIXADDR_TOP, "Fixmap end" },
> { -1, NULL },
> };
>
> --
> 2.39.2
>
>