Re: [PATCH 4/5] mm/swap_state: fix potential faulted in race in swap_ra_info()

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:

> While we released the pte lock, somebody else might faulted in this pte.
> So we should check whether it's swap pte first to guard against such race
> or swp_type would be unexpected. And we can also avoid some unnecessary
> readahead cpu cycles possibly.
>
> Fixes: ec560175c0b6 ("mm, swap: VMA based swap readahead")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  mm/swap_state.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 709c260d644a..3bf0d0c297bc 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -724,10 +724,10 @@ static void swap_ra_info(struct vm_fault *vmf,
>  {
>  	struct vm_area_struct *vma = vmf->vma;
>  	unsigned long ra_val;
> -	swp_entry_t entry;
> +	swp_entry_t swap_entry;
>  	unsigned long faddr, pfn, fpfn;
>  	unsigned long start, end;
> -	pte_t *pte, *orig_pte;
> +	pte_t *pte, *orig_pte, entry;
>  	unsigned int max_win, hits, prev_win, win, left;
>  #ifndef CONFIG_64BIT
>  	pte_t *tpte;
> @@ -742,8 +742,13 @@ static void swap_ra_info(struct vm_fault *vmf,
>  
>  	faddr = vmf->address;
>  	orig_pte = pte = pte_offset_map(vmf->pmd, faddr);
> -	entry = pte_to_swp_entry(*pte);
> -	if ((unlikely(non_swap_entry(entry)))) {
> +	entry = *pte;
> +	if (unlikely(!is_swap_pte(entry))) {
> +		pte_unmap(orig_pte);
> +		return;
> +	}
> +	swap_entry = pte_to_swp_entry(entry);
> +	if ((unlikely(non_swap_entry(swap_entry)))) {
>  		pte_unmap(orig_pte);
>  		return;
>  	}

This isn't a real issue.  entry or swap_entry isn't used in this
function.  And we have enough checking when we really operate the PTE
entries later.  But I admit it's confusing.  So I suggest to just remove
the checking.  We will check it when necessary.

Best Regards,
Huang, Ying