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() -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
