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    SubjectRe: [PATCH -mm -v5 RESEND] mm, swap: Fix race between swapoff and some swap operations
    On Wed, Feb 21, 2018 at 7:38 AM, Andrew Morton
    <akpm@linux-foundation.org> wrote:
    > 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?

    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

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