Re: [PATCH v3 2/9] riscv: mm: Pre-allocate vmemmap/direct map PGD entries

["Björn Töpel" <bjorn@kernel.org>]

Björn Töpel <bjorn@kernel.org> writes:

> From: Björn Töpel <bjorn@rivosinc.com>
>
> The RISC-V port copies the PGD table from init_mm/swapper_pg_dir to
> all userland page tables, which means that if the PGD level table is
> changed, other page tables has to be updated as well.
>
> Instead of having the PGD changes ripple out to all tables, the
> synchronization can be avoided by pre-allocating the PGD entries/pages
> at boot, avoiding the synchronization all together.
>
> This is currently done for the bpf/modules, and vmalloc PGD regions.
> Extend this scheme for the PGD regions touched by memory hotplugging.
>
> Prepare the RISC-V port for memory hotplug by pre-allocate
> vmemmap/direct map entries at the PGD level. This will roughly waste
> ~128 worth of 4K pages when memory hotplugging is enabled in the
> kernel configuration.
>
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
> ---
>  arch/riscv/include/asm/kasan.h | 4 ++--
>  arch/riscv/mm/init.c           | 7 +++++++
>  2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/kasan.h b/arch/riscv/include/asm/kasan.h
> index 0b85e363e778..e6a0071bdb56 100644
> --- a/arch/riscv/include/asm/kasan.h
> +++ b/arch/riscv/include/asm/kasan.h
> @@ -6,8 +6,6 @@
>  
>  #ifndef __ASSEMBLY__
>  
> -#ifdef CONFIG_KASAN
> -
>  /*
>   * The following comment was copied from arm64:
>   * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
> @@ -34,6 +32,8 @@
>   */
>  #define KASAN_SHADOW_START	((KASAN_SHADOW_END - KASAN_SHADOW_SIZE) & PGDIR_MASK)
>  #define KASAN_SHADOW_END	MODULES_LOWEST_VADDR
> +
> +#ifdef CONFIG_KASAN
>  #define KASAN_SHADOW_OFFSET	_AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
>  
>  void kasan_init(void);
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index b66f846e7634..c98010ede810 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -27,6 +27,7 @@
>  
>  #include <asm/fixmap.h>
>  #include <asm/io.h>
> +#include <asm/kasan.h>
>  #include <asm/numa.h>
>  #include <asm/pgtable.h>
>  #include <asm/sections.h>
> @@ -1488,10 +1489,16 @@ static void __init preallocate_pgd_pages_range(unsigned long start, unsigned lon
>  	panic("Failed to pre-allocate %s pages for %s area\n", lvl, area);
>  }
>  
> +#define PAGE_END KASAN_SHADOW_START
> +
>  void __init pgtable_cache_init(void)
>  {
>  	preallocate_pgd_pages_range(VMALLOC_START, VMALLOC_END, "vmalloc");
>  	if (IS_ENABLED(CONFIG_MODULES))
>  		preallocate_pgd_pages_range(MODULES_VADDR, MODULES_END, "bpf/modules");
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
> +		preallocate_pgd_pages_range(VMEMMAP_START, VMEMMAP_END, "vmemmap");
> +		preallocate_pgd_pages_range(PAGE_OFFSET, PAGE_END, "direct map");

Alex pointed out that KASAN PGDs should be preallocated as well! I'll
address this in the next revision.


Björn