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
