From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754399AbcKNPxC (ORCPT ); Mon, 14 Nov 2016 10:53:02 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:47708 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932109AbcKNPxA (ORCPT ); Mon, 14 Nov 2016 10:53:00 -0500 Date: Mon, 14 Nov 2016 10:52:50 -0500 From: Johannes Weiner To: "Kirill A. Shutemov" Cc: Jan Kara , Andrew Morton , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem collapse error path Message-ID: <20161114155250.GB3291@cmpxchg.org> References: <20161107190741.3619-1-hannes@cmpxchg.org> <20161107190741.3619-2-hannes@cmpxchg.org> <20161108095352.GH32353@quack2.suse.cz> <20161108161245.GA4020@cmpxchg.org> <20161111105921.GC19382@node.shutemov.name> <20161111122224.GA5090@quack2.suse.cz> <20161111163753.GH19382@node.shutemov.name> <20161114080744.GA2524@quack2.suse.cz> <20161114142902.GA10455@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161114142902.GA10455@node.shutemov.name> User-Agent: Mutt/1.7.1 (2016-10-04) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 14, 2016 at 05:29:02PM +0300, Kirill A. Shutemov wrote: > On Mon, Nov 14, 2016 at 09:07:44AM +0100, Jan Kara wrote: > > On Fri 11-11-16 19:37:53, Kirill A. Shutemov wrote: > > > 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 > > > > > > > > Signed-off-by: Johannes Weiner > > > > > > > > --- > > > > > > > > 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? > > > > Full lookup is the only way to re-validate the slot. There is no way to pin > > a radix tree node. > > I guess this should be enough: > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 728d7790dc2d..c5ef73588676 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1400,7 +1400,9 @@ static void collapse_shmem(struct mm_struct *mm, > PAGE_SIZE, 0); > > spin_lock_irq(&mapping->tree_lock); > - > + slot = radix_tree_lookup_slot(&mapping->page_tree, index); > + VM_BUG_ON_PAGE(page != radix_tree_deref_slot_protected(slot, > + &mapping->tree_lock), page); > VM_BUG_ON_PAGE(page_mapped(page), page); That looks good to me. The slot may get relocated, but the content shouldn't change with the page locked. Are you going to send a full patch with changelog and sign-off? If so, please add: Acked-by: Johannes Weiner