Re: [PATCH v4 09/45] x86: kmsan: pgtable: reduce vmalloc space

Marco Elver
2022-07-11T16:13:46Z

On Fri, 1 Jul 2022 at 16:23, Alexander Potapenko <glider@google.com> wrote:
>
> KMSAN is going to use 3/4 of existing vmalloc space to hold the
> metadata, therefore we lower VMALLOC_END to make sure vmalloc() doesn't
> allocate past the first 1/4.
>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> ---
> v2:
> -- added x86: to the title
>
> Link: https://linux-review.googlesource.com/id/I9d8b7f0a88a639f1263bc693cbd5c136626f7efd
> ---
> arch/x86/include/asm/pgtable_64_types.h | 41 ++++++++++++++++++++++++-
> arch/x86/mm/init_64.c | 2 +-
> 2 files changed, 41 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 70e360a2e5fb7..ad6ded5b1dedf 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -139,7 +139,46 @@ extern unsigned int ptrs_per_p4d;
> # define VMEMMAP_START __VMEMMAP_BASE_L4
> #endif /* CONFIG_DYNAMIC_MEMORY_LAYOUT */
>
> -#define VMALLOC_END (VMALLOC_START + (VMALLOC_SIZE_TB << 40) - 1)
> +#define VMEMORY_END (VMALLOC_START + (VMALLOC_SIZE_TB << 40) - 1)

Comment what VMEMORY_END is? (Seems obvious, but less guessing is better here.)

> +#ifndef CONFIG_KMSAN
> +#define VMALLOC_END VMEMORY_END
> +#else
> +/*
> + * In KMSAN builds vmalloc area is four times smaller, and the remaining 3/4
> + * are used to keep the metadata for virtual pages. The memory formerly
> + * belonging to vmalloc area is now laid out as follows:
> + *
> + * 1st quarter: VMALLOC_START to VMALLOC_END - new vmalloc area
> + * 2nd quarter: KMSAN_VMALLOC_SHADOW_START to
> + * VMALLOC_END+KMSAN_VMALLOC_SHADOW_OFFSET - vmalloc area shadow
> + * 3rd quarter: KMSAN_VMALLOC_ORIGIN_START to
> + * VMALLOC_END+KMSAN_VMALLOC_ORIGIN_OFFSET - vmalloc area origins
> + * 4th quarter: KMSAN_MODULES_SHADOW_START to KMSAN_MODULES_ORIGIN_START
> + * - shadow for modules,
> + * KMSAN_MODULES_ORIGIN_START to
> + * KMSAN_MODULES_ORIGIN_START + MODULES_LEN - origins for modules.
> + */
> +#define VMALLOC_QUARTER_SIZE ((VMALLOC_SIZE_TB << 40) >> 2)
> +#define VMALLOC_END (VMALLOC_START + VMALLOC_QUARTER_SIZE - 1)
> +
> +/*
> + * vmalloc metadata addresses are calculated by adding shadow/origin offsets
> + * to vmalloc address.
> + */
> +#define KMSAN_VMALLOC_SHADOW_OFFSET VMALLOC_QUARTER_SIZE
> +#define KMSAN_VMALLOC_ORIGIN_OFFSET (VMALLOC_QUARTER_SIZE << 1)
> +
> +#define KMSAN_VMALLOC_SHADOW_START (VMALLOC_START + KMSAN_VMALLOC_SHADOW_OFFSET)
> +#define KMSAN_VMALLOC_ORIGIN_START (VMALLOC_START + KMSAN_VMALLOC_ORIGIN_OFFSET)
> +
> +/*
> + * The shadow/origin for modules are placed one by one in the last 1/4 of
> + * vmalloc space.
> + */
> +#define KMSAN_MODULES_SHADOW_START (VMALLOC_END + KMSAN_VMALLOC_ORIGIN_OFFSET + 1)
> +#define KMSAN_MODULES_ORIGIN_START (KMSAN_MODULES_SHADOW_START + MODULES_LEN)
> +#endif /* CONFIG_KMSAN */
>
> #define MODULES_VADDR (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
> /* The module sections ends with the start of the fixmap */
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 39c5246964a91..5806331172361 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1287,7 +1287,7 @@ static void __init preallocate_vmalloc_pages(void)
> unsigned long addr;
> const char *lvl;
>
> - for (addr = VMALLOC_START; addr <= VMALLOC_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
> + for (addr = VMALLOC_START; addr <= VMEMORY_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
> pgd_t *pgd = pgd_offset_k(addr);
> p4d_t *p4d;
> pud_t *pud;
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>