[Devel] [PATCH rh7] tswap: Add support for zero-filled pages

[Andrey Ryabinin]

On 09/04/2017 03:03 PM, Andrey Ryabinin wrote:
> 
> 
> On 09/04/2017 02:52 PM, Kirill Tkhai wrote:
>> On 31.08.2017 16:45, Andrey Ryabinin wrote:
>>> On 08/03/2017 12:54 PM, Kirill Tkhai wrote:
>>>>  static int tswap_frontswap_store(unsigned type, pgoff_t offset,
>>>>  				 struct page *page)
>>>>  {
>>>>  	swp_entry_t entry = swp_entry(type, offset);
>>>> +	int zero_filled = -1, err = 0;
>>>>  	struct page *cache_page;
>>>> -	int err = 0;
>>>>  
>>>>  	if (!tswap_active)
>>>>  		return -1;
>>>>  
>>>>  	cache_page = tswap_lookup_page(entry);
>>>> -	if (cache_page)
>>>> -		goto copy;
>>>> +	if (cache_page) {
>>>> +		zero_filled = is_zero_filled_page(page);
>>>> +		/* If type of page has not changed, just reuse it */
>>>> +		if (zero_filled == (cache_page == ZERO_PAGE(0)))
>>>> +			goto copy;
>>>> +		tswap_delete_page(entry, NULL);
>>>> +		put_page(cache_page);
>>>
>>> I think if we race with tswap_frontswap_load() this will lead to double put_page().
>>
>> Hm, then if load and store may be executed in parallel, doesn't the current code have the following problem?
>>
> 
> 
> I don't think so.
> 
>> tswap_frontswap_store()                             tswap_frontswap_load()
>>    cache_page = tswap_lookup_page(entry);           ...
>>    goto copy;                                       ...
>>    ...                                              cache_page = tswap_delete_page()
> 
> 						cache_page will be NULL here and tswap_frontswap_load() will bail out without putting page.

Nevermind, I'm talking nonsense.

tswap_frontswap_store(), tswap_frontswap_load() indeed shouldn't run in parallel, so the code should be fine.



>>    ...                                              put_page(cache_page);
>>    ...                                              ...
>>    copy_highpage(cache_page, page);                 ...
>>
>>    ^^^ copy to freed page?
>>
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