Re: [PATCH -mm -v5 RESEND] mm, swap: Fix race between swapoff and some swap operations

[huang ying]

On Wed, Feb 21, 2018 at 7:38 AM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, 18 Feb 2018 09:06:47 +0800 huang ying <huang.ying.caritas@xxxxxxxxx> wrote:
>
>> >> >> +struct swap_info_struct *get_swap_device(swp_entry_t entry)
>> >> >> +{
>> >> >> +  struct swap_info_struct *si;
>> >> >> +  unsigned long type, offset;
>> >> >> +
>> >> >> +  if (!entry.val)
>> >> >> +          goto out;
>> >> >> +  type = swp_type(entry);
>> >> >> +  if (type >= nr_swapfiles)
>> >> >> +          goto bad_nofile;
>> >> >> +  si = swap_info[type];
>> >> >> +
>> >> >> +  preempt_disable();
>> >> >
>> >> > This preempt_disable() is later than I'd expect.  If a well-timed race
>> >> > occurs, `si' could now be pointing at a defunct entry.  If that
>> >> > well-timed race include a swapoff AND a swapon, `si' could be pointing
>> >> > at the info for a new device?
>> >>
>> >> struct swap_info_struct pointed to by swap_info[] will never be freed.
>> >> During swapoff, we only free the memory pointed to by the fields of
>> >> struct swap_info_struct.  And when swapon, we will always reuse
>> >> swap_info[type] if it's not NULL.  So it should be safe to dereference
>> >> swap_info[type] with preemption enabled.
>> >
>> > That's my point.  If there's a race window during which there is a
>> > parallel swapoff+swapon, this swap_info_struct may now be in use for a
>> > different device?
>>
>> Yes.  It's possible.  And the caller of get_swap_device() can live
>> with it if the swap_info_struct has been fully initialized.  For
>> example, for the race in the patch description,
>>
>> do_swap_page
>>   swapin_readahead
>>     __read_swap_cache_async
>>       swapcache_prepare
>>         __swap_duplicate
>>
>> in __swap_duplicate(), it's possible that the swap device returned by
>> get_swap_device() is different from the swap device when
>> __swap_duplicate() call get_swap_device().  But the struct_info_struct
>> has been fully initialized, so __swap_duplicate() can reference
>> si->swap_map[] safely.  And we will check si->swap_map[] before any
>> further operation.  Even if the swap entry is swapped out again for
>> the new swap device, we will check the page table again in
>> do_swap_page().  So there is no functionality problem.
>
> That's rather revolting.  Can we tighten this up?  Or at least very
> loudly document it?

TBH, I think my original fix patch which uses a reference count in
swap_info_struct is easier to be understood.  But I understand it has
its own drawbacks too.  Anyway, unless there are some better ideas to
resolve this, I will send out a new version with more document.

Best Regards,
Huang, Ying