Re: [PATCH] mm, swap: Remove unnecessary smp_rmb() in swap_type_to_swap_info()

[Huang, Ying]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Thu, May 13, 2021 at 02:46:10PM +0200, Peter Zijlstra wrote:
>> Ah, I think I see what you meant to say, it would perhaps help if you
>> write it like so:
>> 
>> 
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 149e77454e3c..94735248dcd2 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -99,11 +99,10 @@ atomic_t nr_rotate_swap = ATOMIC_INIT(0);
>>  
>>  static struct swap_info_struct *swap_type_to_swap_info(int type)
>>  {
>> -	if (type >= READ_ONCE(nr_swapfiles))
>> +	if (type >= MAX_SWAPFILES)
>>  		return NULL;
>>  
>> -	smp_rmb();	/* Pairs with smp_wmb in alloc_swap_info. */
>> -	return READ_ONCE(swap_info[type]);
>> +	return READ_ONCE(swap_info[type]); /* rcu_dereference() */
>>  }
>>  
>>  static inline unsigned char swap_count(unsigned char ent)
>> @@ -2869,14 +2868,11 @@ static struct swap_info_struct *alloc_swap_info(void)
>>  	}
>>  	if (type >= nr_swapfiles) {
>>  		p->type = type;
>> -		WRITE_ONCE(swap_info[type], p);
>>  		/*
>> -		 * Write swap_info[type] before nr_swapfiles, in case a
>> -		 * racing procfs swap_start() or swap_next() is reading them.
>> -		 * (We never shrink nr_swapfiles, we never free this entry.)
>> +		 * Publish the swap_info_struct.
>>  		 */
>> -		smp_wmb();
>> -		WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
>> +		smp_store_release(&swap_info[type], p); /* rcu_assign_pointer() */
>> +		nr_swapfiles++;
>
> Yes, this does help, I didn't understand why smp_wmb stayed around in
> the original post.
>
> I think the only access smp_store_release() orders is p->type.  Wouldn't
> it be kinda inconsistent to only initialize that one field before
> publishing when many others would be done at the end of
> alloc_swap_info() after the fact?

In addition to p->type, *p is zeroed via kvzalloc().

> p->type doesn't seem special.  For
> instance, get_swap_page_of_type() touches si->lock soon after it calls
> swap_type_to_swap_info(), so there could be a small window where there's
> a non-NULL si with an uninitialized lock.

We usually check the state of swap_info_struct before other operations.
For example, we check si->swap_map in swap_start().

> It's not as if this is likely to be a problem in practice, it would just
> make it harder to understand why smp_store_release is there.  Maybe all
> we need is a WRITE_ONCE, or if it's really necessary for certain fields
> to be set before publication then move them up and explain?

I think we have initialized all fields before publication :-).

Best Regards,
Huang, Ying