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

[huang ying <huang.ying.caritas@gmail.com>]

On Sat, Feb 17, 2018 at 7:38 AM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 14 Feb 2018 08:38:00 +0800 "Huang\, Ying" <ying.huang@intel.com> wrote:
>
>> Andrew Morton <akpm@linux-foundation.org> writes:
>>
>> > On Tue, 13 Feb 2018 09:42:20 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:
>> >
>> >> From: Huang Ying <ying.huang@intel.com>
>> >>
>> >> When the swapin is performed, after getting the swap entry information
>> >> from the page table, system will swap in the swap entry, without any
>> >> lock held to prevent the swap device from being swapoff.  This may
>> >> cause the race like below,
>> >
>> > Sigh.  In terms of putting all the work into the swapoff path and
>> > avoiding overheads in the hot paths, I guess this is about as good as
>> > it will get.
>> >
>> > It's a very low-priority fix so I'd prefer to keep the patch in -mm
>> > until Hugh has had an opportunity to think about it.
>> >
>> >> ...
>> >>
>> >> +/*
>> >> + * Check whether swap entry is valid in the swap device.  If so,
>> >> + * return pointer to swap_info_struct, and keep the swap entry valid
>> >> + * via preventing the swap device from being swapoff, until
>> >> + * put_swap_device() is called.  Otherwise return NULL.
>> >> + */
>> >> +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.

Best Regards,
Huang, Ying