On Fri, Mar 08, 2013 at 10:48:31AM +0800, Simon Jeons wrote: > On 03/08/2013 10:37 AM, Johannes Weiner wrote: > >On Fri, Mar 08, 2013 at 10:13:25AM +0800, Simon Jeons wrote: > >>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? > >It doesn't because the pagevec holds a reference, as I wrote above. > > I mean since pagevec is per cpu, how can remove a concurrently freed > page out of a pagevec happen? If it doesn't happen pagevec don't > need to hold a reference. :-) It has nothing to do with the pagevec being per CPU. The page may get truncated or reclaimed and have every other reference being dropped while it sits on the pagevec. -- 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>