[PATCH] mm: Fixup the condition whether the page cache is free

[Li Haifeng]

Thanks very much for you explanation. :-)

2013/3/4 Johannes Weiner <hannes at cmpxchg.org>:
> On Mon, Mar 04, 2013 at 09:54:26AM +0800, Li Haifeng wrote:
>> When a page cache is to reclaim, we should to decide whether the page
>> cache is free.
>> IMO, the condition whether a page cache is free should be 3 in page
>> frame reclaiming. The reason lists as below.
>>
>> When page is allocated, the page->_count is 1(code fragment is code-1 ).
>> And when the page is allocated for reading files from extern disk, the
>> page->_count will increment 1 by page_cache_get() in
>> add_to_page_cache_locked()(code fragment is code-2). When the page is to
>> reclaim, the isolated LRU list also increase the page->_count(code
>> fragment is code-3).
>
> The page count is initialized to 1, but that does not stay with the
> object.  It's a reference that is passed to the allocating task, which
> drops it again when it's done with the page.  I.e. the pattern is like
> this:
>
> instantiation:
> page = page_cache_alloc()       /* instantiator reference -> 1 */
> add_to_page_cache(page, mapping, offset)
>   get_page(page)                /* page cache reference -> 2 */
> lru_cache_add(page)
>   get_page(page)                /* pagevec reference -> 3 */
> /* ...initiate read, write, associate buffers, ... */
> page_cache_release(page)        /* drop instantiator reference -> 2 + private */
>
> reclaim:
> lru_add_drain()
>   page_cache_release(page)      /* drop pagevec reference -> 1 + private */
> __isolate_lru_page(page)
>   page_cache_get(page)          /* reclaim reference -> 2 + private */
> is_page_cache_freeable(page)
> try_to_free_buffers()           /* drop buffer ref -> 2 */
> __remove_mapping()              /* drop page cache and isolator ref -> 0 */
> free_hot_cold_page()