Re: [PATCH] mm/swap_slots.c: don't reset the cache slot after use

[Tim Chen]

On 3/9/20 5:48 PM, Andrew Morton wrote:
> On Mon,  9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx> wrote:
> 
>> Currently we would clear the cache slot if it is used. While this is not
>> necessary, since this entry would not be used until refilled.
>>
>> Leave it untouched and assigned the value directly to entry which makes
>> the code little more neat.
>>
>> Also this patch merges the else and if, since this is the only case we
>> refill and repeat swap cache.
> 
> cc Tim, who can hopefully remember how this code works ;)
> 
>> --- a/mm/swap_slots.c
>> +++ b/mm/swap_slots.c
>> @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry)
>>  
>>  swp_entry_t get_swap_page(struct page *page)
>>  {
>> -	swp_entry_t entry, *pentry;
>> +	swp_entry_t entry;
>>  	struct swap_slots_cache *cache;
>>  
>>  	entry.val = 0;
>> @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page)
>>  		if (cache->slots) {
>>  repeat:
>>  			if (cache->nr) {
>> -				pentry = &cache->slots[cache->cur++];
>> -				entry = *pentry;
>> -				pentry->val = 0;

The cache entry was cleared after assignment for defensive programming,  So there's
little chance I will be using a slot that has been assigned to someone else.
When I wrote swap_slots.c, this code was new and I want to make sure
that if something went wrong, and I assigned a swap slot that I shouldn't,
I will be able to detect quickly as I will only be stepping on entry 0.

Otherwise such bug will be harder to detect as we will have two users of some random
swap slot stepping on each other.

I'm okay if we want to get rid of this logic, now that the code has been
working correctly long enough.  But I think is good hygiene to clear the
cached entry after it has been assigned. 

>> +				entry = cache->slots[cache->cur++];
>>  				cache->nr--;
>> -			} else {
>> -				if (refill_swap_slots_cache(cache))
>> -					goto repeat;
>> +			} else if (refill_swap_slots_cache(cache)) {

This change looks fine.
>> +				goto repeat;
>>  			}

Tim