Re: [syzbot] memory leak in xas_create

[Matthew Wilcox]

On Tue, Jul 12, 2022 at 02:50:50PM +0200, Dmitry Vyukov wrote:
> On Tue, 12 Jul 2022 at 14:40, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Tue, Jul 12, 2022 at 08:54:28AM +0200, Dmitry Vyukov wrote:
> > > On Mon, 11 Jul 2022 at 22:47, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Jul 11, 2022 at 01:38:08PM -0700, Andrew Morton wrote:
> > > > > On Sat, 09 Jul 2022 00:13:23 -0700 syzbot <syzbot+a785d07959bc94837d51@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > > Hello,
> > > > > >
> > > > > > syzbot found the following issue on:
> > > > > >
> > > > > > HEAD commit:    c1084b6c5620 Merge tag 'soc-fixes-5.19-2' of git://git.ker..
> > > > > > git tree:       upstream
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=14967ccc080000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=916233b7694a38ff
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=a785d07959bc94837d51
> > > > > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=122ae834080000
> > > > > >
> > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > > > Reported-by: syzbot+a785d07959bc94837d51@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > > >
> > > > > > 2022/07/05 05:22:17 executed programs: 828
> > > > > > 2022/07/05 05:22:23 executed programs: 846
> > > > > > 2022/07/05 05:22:30 executed programs: 866
> > > > > > 2022/07/05 05:22:37 executed programs: 875
> > > > > > BUG: memory leak
> > > > >
> > > > > Thanks.  Presumably due to khugepaged changes.
> > > >
> > > > Huh, I was expecting it to be something I'd messed up.  I've been
> > > > looking at it today, but no luck figuring it out so far.
> > > >
> > > > > Can we expect a bisection search?
> > > >
> > > > We only have a syz reproducer so far, and if I understand correctly,
> > > > it's probably because this is a flaky test (because it's trying to
> > > > find something that's a race condition).
> > > >
> > > > I expect a bisection search to go badly wrong if this is true.
> > >
> > > Is it possible that parts of xas are not freed on the error paths?
> > > I don't immediately see where anything is freed on these error paths:
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/xarray.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n681
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/xarray.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n721
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/khugepaged.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n1675
> >
> > There's nothing to free; if a node is allocated, then it's stored in
> > the tree where it can later be found and reused.
> 
> What I was thinking of is:
> 
> The leaked memory is allocated with:
> xas_create_range(&xas);
> here:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/khugepaged.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n1670
> 
> So I assumed the nodes stored in the xas object, which is local to the
> collapse_file() function.

Yes, that's a reasonable thing to think, but it's actually not how
it works.  When we allocate a node in xas_create(), we put it straight
into the tree without storing it in xas->xa_alloc.  We may then end
up not using it, but the node isn't leaked because it's in the tree.

If the GFP_NOWAIT allocation fails (it didn't in these stack traces),
we call xas_nomem(), which sees an -ENOMEM, allocates a node and stores
it in xas->xa_alloc; then we go round the loop again where xas_create()
will take the node from xas->xa_alloc.  But the backtraces here don't
implicate xas_nomem().

> So if we do "goto out" here:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/khugepaged.c?id=c1084b6c5620a743f86947caca66d90f24060f56#n1676
> 
> There does not seem to be anything that frees anything stored in the xas:
> 
> out:
>     VM_BUG_ON(!list_empty(&pagelist));
>     if (!IS_ERR_OR_NULL(*hpage))
>         mem_cgroup_uncharge(page_folio(*hpage));
>     /* TODO: tracepoints */
> }
>