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

[Wei Yang]

On Tue, Mar 10, 2020 at 11:13:13AM -0700, Tim Chen wrote:
>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. 
>

This is fine to keep the logic, while I am wondering whether we need to do
this through pointer. cache->slots[] contain the value, we can get and reset
without pointer.

The following code looks more obvious about the logic.

		entry = cache->slots[cache->cur];
		cache->slots[cache->cur++].val = 0;


>>> +				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

-- 
Wei Yang
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