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

[Kirill Tkhai]

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?

tswap_frontswap_store()                             tswap_frontswap_load()
   cache_page = tswap_lookup_page(entry);           ...
   goto copy;                                       ...
   ...                                              cache_page = tswap_delete_page()
   ...                                              put_page(cache_page);
   ...                                              ...
   copy_highpage(cache_page, page);                 ...

   ^^^ copy to freed page?