On Sat, 26 Jan 2013, Simon Jeons wrote: > On Fri, 2013-01-25 at 18:01 -0800, Hugh Dickins wrote: > > Switching merge_across_nodes after running KSM is liable to oops on stale > > nodes still left over from the previous stable tree. It's not something > > that people will often want to do, but it would be lame to demand a reboot > > when they're trying to determine which merge_across_nodes setting is best. > > > > How can this happen? We only permit switching merge_across_nodes when > > pages_shared is 0, and usually set run 2 to force that beforehand, which > > ought to unmerge everything: yet oopses still occur when you then run 1. > > > > Three causes: > > > > 1. The old stable tree (built according to the inverse merge_across_nodes) > > has not been fully torn down. A stable node lingers until get_ksm_page() > > notices that the page it references no longer references it: but the page > > is not necessarily freed as soon as expected, particularly when swapcache. > > > > When can this happen? Whenever there's an additional reference to the page, beyond those for its ptes in userspace - swapcache for example, or pinned by get_user_pages. That delays its being freed (arriving at the "page->mapping = NULL;" in free_pages_prepare()). Or it might simply be sitting in a pagevec, waiting for that to be filled up, to be freed as part of a batch. > > > Fix this with a pass through the old stable tree, applying get_ksm_page() > > to each of the remaining nodes (most found stale and removed immediately), > > with forced removal of any left over. Unless the page is still mapped: > > I've not seen that case, it shouldn't occur, but better to WARN_ON_ONCE > > and EBUSY than BUG. > > > > 2. __ksm_enter() has a nice little optimization, to insert the new mm > > just behind ksmd's cursor, so there's a full pass for it to stabilize > > (or be removed) before ksmd addresses it. Nice when ksmd is running, > > but not so nice when we're trying to unmerge all mms: we were missing > > those mms forked and inserted behind the unmerge cursor. Easily fixed > > by inserting at the end when KSM_RUN_UNMERGE. > > mms forked will be unmerged just after ksmd's cursor since they're > inserted behind it, why will be missing? unmerge_and_remove_all_rmap_items() makes one pass through the list from start to finish: insert behind the cursor and it will be missed. -- 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>