Re: [PATCH] mm, mempolicy: fix up gup usage in lookup_node

[Peter Xu]

On Tue, Apr 21, 2020 at 09:10:26AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal") has
> added a special casing for 0 return value because that was a possible
> gup return value when interrupted by fatal signal. This has been fixed
> by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR
> for fatal signal") in the mean time so ba841078cd05 can be reverted.
> 
> This patch however doesn't go all the way to revert it because the check
> for 0 is wrong and confusing here. Firstly it is inherently unsafe to
> access the page when get_user_pages_locked returns 0 (aka no page
> returned).
> Fortunatelly this will not happen because get_user_pages_locked will not
> return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not
> the case here. Document this potential error code in gup code while we
> are at it.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/gup.c       | 5 +++++
>  mm/mempolicy.c | 5 +----
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0286de..a8575b880baf 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -980,6 +980,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>   * -- If nr_pages is >0, but no pages were pinned, returns -errno.
>   * -- If nr_pages is >0, and some pages were pinned, returns the number of
>   *    pages pinned. Again, this may be less than nr_pages.
> + * -- 0 return value is possible when the fault would need to be retried.
>   *
>   * The caller is responsible for releasing returned @pages, via put_page().
>   *
> @@ -1247,6 +1248,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
>  }
>  EXPORT_SYMBOL_GPL(fixup_user_fault);
>  
> +/*
> + * Please note that this function, unlike __get_user_pages will not
> + * return 0 for nr_pages > 0 without FOLL_NOWAIT

It's a bit unclear to me on whether "will not return 0" applies to "this
function" or "__get_user_pages"...  Might be easier just to avoid mentioning
__get_user_pages?

> + */
>  static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
>  						struct mm_struct *mm,
>  						unsigned long start,
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 48ba9729062e..1965e2681877 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr)
>  
>  	int locked = 1;
>  	err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked);
> -	if (err == 0) {
> -		/* E.g. GUP interrupted by fatal signal */
> -		err = -EFAULT;
> -	} else if (err > 0) {
> +	if (err > 0) {
>  		err = page_to_nid(p);
>  		put_page(p);
>  	}

Again, this is my totally humble opinion: I'm fine with removing the comment,
however I still don't think it's helpful at all to explicitly remove a check
against invalid return value (err==0), especially if that's the only functional
change in this patch.

Thanks,

-- 
Peter Xu