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

[simon.jeons@gmail.com (Simon Jeons)]

Ping, :-)
On 03/07/2013 09:05 AM, Simon Jeons wrote:
> Hi Johannes,
> On 03/07/2013 03:47 AM, Johannes Weiner wrote:
>> On Wed, Mar 06, 2013 at 09:04:55AM +0800, Simon Jeons wrote:
>>> Hi Johannes,
>>> On 03/04/2013 11:09 PM, Johannes Weiner wrote:
>>>> 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 */
>>> IIUC, when add page to lru will lead to add to pagevec firstly, and
>>> pagevec will take one reference, so if lru will take over the
>>> reference taken by pagevec when page transmit from pagevec to lru?
>>> or just drop the reference and lru will not take reference for page?
>> The LRU does not hold a reference, it would not make sense.  The
>> pagevec only needs one because it would be awkward to remove a
>> concurrently freed page out of a pagevec, but unlinking a page from
>> the LRU is easy.  See mm/swap.c::__page_cache_release() and friends.
>
> Since pagevec is per cpu, when can remove a concurrently freed page 
> out of a pagevec happen?
>
>