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

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 18 Feb 2018 09:06:47 +0800 huang ying <huang.ying.caritas@gmail.com> 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?