Messages in this thread | | | Date | Sun, 27 Jan 2013 14:08:00 -0800 (PST) | From | Hugh Dickins <> | Subject | Re: [PATCH 5/11] ksm: get_ksm_page locked |
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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.
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