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Date: Fri, 11 Nov 2016 13:59:21 +0300 From: "Kirill A. Shutemov" <kirill@...temov.name> To: Johannes Weiner <hannes@...xchg.org> Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...com Subject: Re: [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem collapse error path 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 <jack@...e.cz> > > > Signed-off-by: Johannes Weiner <hannes@...xchg.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. > I'll update this patch and prepend another fix to the series that > addresses the other two lock dropping issues. Feel free add my Acked-by. -- Kirill A. Shutemov
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