On 2021/4/19 10:23, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>
>> When I was investigating the swap code, I found the below possible race
>> window:
>>
>> CPU 1 CPU 2
>> ----- -----
>> do_swap_page
>
> This is OK for swap cache cases. So
>
> if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
>
> should be shown here.
Ok.
>
>> swap_readpage(skip swap cache case)
>> if (data_race(sis->flags & SWP_FS_OPS)) {
>> swapoff
>> p->flags = &= ~SWP_VALID;
>> ..
>> synchronize_rcu();
>> ..
>> p->swap_file = NULL;
>> struct file *swap_file = sis->swap_file;
>> struct address_space *mapping = swap_file->f_mapping;[oops!]
>>
>> Note that for the pages that are swapped in through swap cache, this isn't
>> an issue. Because the page is locked, and the swap entry will be marked
>> with SWAP_HAS_CACHE, so swapoff() can not proceed until the page has been
>> unlocked.
>>
>> Using current get/put_swap_device() to guard against concurrent swapoff for
>> swap_readpage() looks terrible because swap_readpage() may take really long
>> time. And this race may not be really pernicious because swapoff is usually
>> done when system shutdown only. To reduce the performance overhead on the
>> hot-path as much as possible, it appears we can use the percpu_ref to close
>> this race window(as suggested by Huang, Ying).
>
> I still suggest to squash PATCH 1-3, at least PATCH 1-2. That will
> change the relevant code together and make it easier to review.
>
Will squash PATCH 1-2. Thanks.
> Best Regards,
> Huang, Ying
>
>> Fixes: 0bcac06f27d7 ("mm,swap: skip swapcache for swapin of synchronous device")
>> Reported-by: kernel test robot <lkp@xxxxxxxxx> (auto build test ERROR)
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>> include/linux/swap.h | 9 +++++++++
>> mm/memory.c | 9 +++++++++
>> 2 files changed, 18 insertions(+)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 993693b38109..523c2411a135 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -528,6 +528,15 @@ static inline struct swap_info_struct *swp_swap_info(swp_entry_t entry)
>> return NULL;
>> }
>>
>> +static inline struct swap_info_struct *get_swap_device(swp_entry_t entry)
>> +{
>> + return NULL;
>> +}
>> +
>> +static inline void put_swap_device(struct swap_info_struct *si)
>> +{
>> +}
>> +
>> #define swap_address_space(entry) (NULL)
>> #define get_nr_swap_pages() 0L
>> #define total_swap_pages 0L
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 27014c3bde9f..7a2fe12cf641 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3311,6 +3311,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> {
>> struct vm_area_struct *vma = vmf->vma;
>> struct page *page = NULL, *swapcache;
>> + struct swap_info_struct *si = NULL;
>> swp_entry_t entry;
>> pte_t pte;
>> int locked;
>> @@ -3338,6 +3339,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> goto out;
>> }
>>
>> + /* Prevent swapoff from happening to us. */
>> + si = get_swap_device(entry);
>> + if (unlikely(!si))
>> + goto out;
>>
>> delayacct_set_flag(current, DELAYACCT_PF_SWAPIN);
>> page = lookup_swap_cache(entry, vma, vmf->address);
>> @@ -3514,6 +3519,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> unlock:
>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>> out:
>> + if (si)
>> + put_swap_device(si);
>> return ret;
>> out_nomap:
>> pte_unmap_unlock(vmf->pte, vmf->ptl);
>> @@ -3525,6 +3532,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> unlock_page(swapcache);
>> put_page(swapcache);
>> }
>> + if (si)
>> + put_swap_device(si);
>> return ret;
>> }
> .
>
