Re: [PATCH v6 08/12] fork: Clear PASID for new mm

[Jean-Philippe Brucker]

Hi Fenghua,

[Trimmed the Cc list]

On Mon, Jul 13, 2020 at 04:48:03PM -0700, Fenghua Yu wrote:
> When a new mm is created, its PASID should be cleared, i.e. the PASID is
> initialized to its init state 0 on both ARM and X86.

I just noticed this patch was dropped in v7, and am wondering whether we
could still upstream it. Does x86 need a child with a new address space
(!CLONE_VM) to inherit the PASID of the parent?  That doesn't make much
sense with regard to IOMMU structures - same PASID indexing multiple PGDs?

Currently iommu_sva_alloc_pasid() assumes mm->pasid is always initialized
to 0 and fails on forked tasks. I'm trying to figure out how to fix this.
Could we clear the pasid on fork or does it break the x86 model?

Thanks,
Jean

> 
> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> v2:
> - Add this patch to initialize PASID value for a new mm.
> 
>  include/linux/mm_types.h | 2 ++
>  kernel/fork.c            | 8 ++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index d61285cfe027..d60d2ec10881 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -22,6 +22,8 @@
>  #endif
>  #define AT_VECTOR_SIZE (2*(AT_VECTOR_SIZE_ARCH + AT_VECTOR_SIZE_BASE + 1))
>  
> +/* Initial PASID value is 0. */
> +#define INIT_PASID	0
>  
>  struct address_space;
>  struct mem_cgroup;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 142b23645d82..43b5f112604d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1007,6 +1007,13 @@ static void mm_init_owner(struct mm_struct *mm, struct task_struct *p)
>  #endif
>  }
>  
> +static void mm_init_pasid(struct mm_struct *mm)
> +{
> +#ifdef CONFIG_IOMMU_SUPPORT
> +	mm->pasid = INIT_PASID;
> +#endif
> +}
> +
>  static void mm_init_uprobes_state(struct mm_struct *mm)
>  {
>  #ifdef CONFIG_UPROBES
> @@ -1035,6 +1042,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  	mm_init_cpumask(mm);
>  	mm_init_aio(mm);
>  	mm_init_owner(mm, p);
> +	mm_init_pasid(mm);
>  	RCU_INIT_POINTER(mm->exe_file, NULL);
>  	mmu_notifier_subscriptions_init(mm);
>  	init_tlb_flush_pending(mm);
> -- 
> 2.19.1
>