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

["Huang\, Ying" <ying.huang@intel.com>]

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.

Best Regards,
Huang, Ying

>> +	if (!(si->flags & SWP_VALID))
>> +		goto unlock_out;
>> +	offset = swp_offset(entry);
>> +	if (offset >= si->max)
>> +		goto unlock_out;
>> +
>> +	return si;
>> +bad_nofile:
>> +	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>> +out:
>> +	return NULL;
>> +unlock_out:
>> +	preempt_enable();
>> +	return NULL;
>> +}