On Fri, Nov 11, 2016 at 01:22:24PM +0100, Jan Kara wrote: > On Fri 11-11-16 13:59:21, Kirill A. Shutemov wrote: > > On Tue, Nov 08, 2016 at 11:12:45AM -0500, Johannes Weiner wrote: > > > On Tue, Nov 08, 2016 at 10:53:52AM +0100, Jan Kara wrote: > > > > On Mon 07-11-16 14:07:36, Johannes Weiner wrote: > > > > > The radix tree counts valid entries in each tree node. Entries stored > > > > > in the tree cannot be removed by simpling storing NULL in the slot or > > > > > the internal counters will be off and the node never gets freed again. > > > > > > > > > > When collapsing a shmem page fails, restore the holes that were filled > > > > > with radix_tree_insert() with a proper radix tree deletion. > > > > > > > > > > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for > > > > > tmpfs/shmem pages") > > > > > Reported-by: Jan Kara <j...@suse.cz> > > > > > Signed-off-by: Johannes Weiner <han...@cmpxchg.org> > > > > > --- > > > > > mm/khugepaged.c | 3 ++- > > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > > > index 728d7790dc2d..eac6f0580e26 100644 > > > > > --- a/mm/khugepaged.c > > > > > +++ b/mm/khugepaged.c > > > > > @@ -1520,7 +1520,8 @@ static void collapse_shmem(struct mm_struct *mm, > > > > > if (!nr_none) > > > > > break; > > > > > /* Put holes back where they were */ > > > > > - radix_tree_replace_slot(slot, NULL); > > > > > + radix_tree_delete(&mapping->page_tree, > > > > > + iter.index); > > > > > > > > Hum, but this is inside radix_tree_for_each_slot() iteration. And > > > > radix_tree_delete() may end up freeing nodes resulting in invalidating > > > > current slot pointer and the iteration code will do use-after-free. > > > > > > Good point, we need to do another tree lookup after the deletion. > > > > > > But there are other instances in the code, where we drop the lock > > > temporarily and somebody else could delete the node from under us. > > > > > > In the main collapse path, I *think* this is prevented by the fact > > > that when we drop the tree lock we still hold the page lock of the > > > regular page that's in the tree while we isolate and unmap it, thus > > > pin the node. Even so, it would seem a little hairy to rely on that. > > > > > > Kirill? > > > > [ sorry for delay ] > > > > Yes, we make sure that locked page still belong to the radix tree and fall > > off if it's not. Locked page cannot be removed from radix-tree, so we > > should be fine. > > Well, it cannot be removed from the radix tree but radix tree code is still > free to collapse / expand the tree nodes as it sees fit (currently the only > real case is when changing direct page pointer in the tree root to a node > pointer or vice versa but still...). So code should not really assume that > the node page is referenced from does not change once tree_lock is dropped. > It leads to subtle bugs...
Hm. Okay. What is the right way re-validate that slot is still valid? Do I need full look up again? Can I pin node explicitly? -- Kirill A. Shutemov