On 2021/4/12 8:55, Huang, Ying wrote:
> 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.
>
I see. Many thanks for explanation. :)
> Best Regards,
> Huang, Ying
> .
>
