On Sat, 26 Jan 2013, Simon Jeons wrote: > On Fri, 2013-01-25 at 18:00 -0800, Hugh Dickins wrote: > > In some places where get_ksm_page() is used, we need the page to be locked. > > > > In function get_ksm_page, why check page->mapping => > get_page_unless_zero => check page->mapping instead of > get_page_unless_zero => check page->mapping, because > get_page_unless_zero is expensive? Yes, it's more expensive. > > > When KSM migration is fully enabled, we shall want that to make sure that > > the page just acquired cannot be migrated beneath us (raised page count is > > only effective when there is serialization to make sure migration notices). > > Whereas when navigating through the stable tree, we certainly do not want > > What's the meaning of "navigating through the stable tree"? Finding the right place in the stable tree, as stable_tree_search() and stable_tree_insert() do. > > > to lock each node (raised page count is enough to guarantee the memcmps, > > even if page is migrated to another node). > > > > Since we're about to add another use case, add the locked argument to > > get_ksm_page() now. > > Why the parameter lock passed from stable_tree_search/insert is true, > but remove_rmap_item_from_tree is false? The other way round? remove_rmap_item_from_tree needs the page locked, because it's about to modify the list: that's secured (e.g. against concurrent KSM page reclaim) by the page lock. stable_tree_search and stable_tree_insert do not need intermediate nodes to be locked: get_page is enough to secure the page contents for memcmp, and we don't want a pointless wait for exclusive page lock on every intermediate node. -- 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>