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.
> 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.
Sounds reasonable. Will do it in v2.
Many thanks for review and reply!
>
> Best Regards,
> Huang, Ying
> .
>
