Re: [PATCH] arm64: Enable vmalloc-huge with ptdump

[Dev Jain <dev.jain@arm.com>]

On 30/05/25 1:50 pm, Dev Jain wrote:
> arm64 disables vmalloc-huge when kernel page table dumping is enabled,
> because an intermediate table may be removed, potentially causing the
> ptdump code to dereference an invalid address. We want to be able to
> analyze block vs page mappings for kernel mappings with ptdump, so to
> enable vmalloc-huge with ptdump, synchronize between page table removal in
> pmd_free_pte_page()/pud_free_pmd_page() and ptdump pagetable walking. We
> use mmap_read_lock and not write lock because we don't need to synchronize
> between two different vm_structs; two vmalloc objects running this same
> code path will point to different page tables, hence there is no race.

I mean, there *is* a race, but there is no problem :)

>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> ---
>   arch/arm64/include/asm/vmalloc.h | 6 ++----
>   arch/arm64/mm/mmu.c              | 7 +++++++
>   2 files changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/include/asm/vmalloc.h b/arch/arm64/include/asm/vmalloc.h
> index 38fafffe699f..28b7173d8693 100644
> --- a/arch/arm64/include/asm/vmalloc.h
> +++ b/arch/arm64/include/asm/vmalloc.h
> @@ -12,15 +12,13 @@ static inline bool arch_vmap_pud_supported(pgprot_t prot)
>   	/*
>   	 * SW table walks can't handle removal of intermediate entries.
>   	 */
> -	return pud_sect_supported() &&
> -	       !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
> +	return pud_sect_supported();
>   }
>   
>   #define arch_vmap_pmd_supported arch_vmap_pmd_supported
>   static inline bool arch_vmap_pmd_supported(pgprot_t prot)
>   {
> -	/* See arch_vmap_pud_supported() */
> -	return !IS_ENABLED(CONFIG_PTDUMP_DEBUGFS);
> +	return true;
>   }
>   
>   #endif
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index ea6695d53fb9..798cebd9e147 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1261,7 +1261,11 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>   	}
>   
>   	table = pte_offset_kernel(pmdp, addr);
> +
> +	/* Synchronize against ptdump_walk_pgd() */
> +	mmap_read_lock(&init_mm);
>   	pmd_clear(pmdp);
> +	mmap_read_unlock(&init_mm);
>   	__flush_tlb_kernel_pgtable(addr);
>   	pte_free_kernel(NULL, table);
>   	return 1;
> @@ -1289,7 +1293,10 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>   		pmd_free_pte_page(pmdp, next);
>   	} while (pmdp++, next += PMD_SIZE, next != end);
>   
> +	/* Synchronize against ptdump_walk_pgd() */
> +	mmap_read_lock(&init_mm);
>   	pud_clear(pudp);
> +	mmap_read_unlock(&init_mm);
>   	__flush_tlb_kernel_pgtable(addr);
>   	pmd_free(NULL, table);
>   	return 1;