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

[Wei Yang <richard.weiyang@linux.alibaba.com>]

On Tue, Mar 10, 2020 at 04:03:07PM -0700, Tim Chen wrote:
>On 3/10/20 3:20 PM, Wei Yang wrote:
>> 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@linux.alibaba.com> 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;
>
>Yes, this looks pretty good.

Thanks, I would rephrase v2.

>
>Thanks.
>
>Tim

-- 
Wei Yang
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