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


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