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

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:

> On 2021/4/9 16:50, Huang, Ying wrote:
>> 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
>
> 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.

Best Regards,
Huang, Ying