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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2021/4/12 8:55, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> On 2021/4/9 16:50, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@huawei.com> 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@huawei.com>
>>>> ---
>>>>  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
>>
>> Agree. It seems the entry or swap_entry here is just used for check whether
>> pte is still valid swap_entry.
> 
> If you check the git history, you will find that the check has been
> necessary before.  Because the function is used earlier in
> do_swap_page() at that time.
> 

I see. Many thanks for explanation. :)

> Best Regards,
> Huang, Ying
> .
>