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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>